Semiconductor device

ABSTRACT

It is an object to provide a semiconductor device which can supply a signal with sufficient amplitude to a scan line while power consumption is kept small. Further, it is an object to provide a semiconductor device which can suppress distortion of a signal supplied to the scan line and shorten a rising time and a falling time while power consumption is kept small. A semiconductor device which includes a plurality of pixels each including a display element and at least one first transistor and a scan line driver circuit supplying a signal for selecting the plurality of pixels to a scan line. A light-transmitting conductive layer is used for a pixel electrode layer of the display element, a gate electrode layer of the first transistor, source and drain electrode layers of the first transistor, and the scan line. The scan line driver circuit includes a second transistor and a capacitor for holding a voltage between a gate electrode layer of the second transistor and a source electrode layer of the second transistor. The source electrode of the second transistor is connected to the scan line.

TECHNICAL FIELD

This invention is related to an active matrix semiconductor device.

BACKGROUND ART

In an active matrix display device, a plurality of scan lines are led tointersect with a plurality of signal lines in a row direction and acolumn direction, and pixels each including a transistor, a pixelelectrode, and a storage capacitor are provided at the intersections.The pixels are controlled by the plurality of scan lines whichsequentially drives the pixels and the plurality of signal lines whichsupplies display signals to pixel electrodes. The scan line is connectedto a scan line driver circuit for controlling the scan line. The signalline is connected to a signal line driver circuit for controlling thesignal line. In order to control a plurality of pixels sequentially, thescan line driver circuit includes as many output terminals as the scanlines. The signal line driver circuit includes as many output terminalsas the signal lines.

Note that, in recent years, a display device has come to have highdefinition and to be larger in size, and it is a problem that powerconsumption is increased as the number of scan lines and signal linesare increased. Meanwhile, reduction in power consumption is highlyneeded. A technique in which power consumption is reduced by reductionof the number of outputs of an external driver circuit is disclosed.

Specifically, there is a technique described in Patent Document 1 below:a plurality of scan line switching element and a scan line drivercircuit including a scan line driver IC and a scan line signal branchcircuit are manufactured, whereby the number of output terminals of thescan line driver IC, so that driving of low power consumption can berealized and the duty ratio of the scan line switching element can bereduced to improve reliability.

REFERENCE

[Patent Document 1] Japanese Published Patent Application No.2002-311879

DISCLOSURE OF INVENTION

In the conventional technique, in the case where a unipolar scan lineswitching element is employed, the amplitude of a scan line selectionsignal is often smaller than that of an output signal of a scan linedriver circuit by the threshold voltage (Vth) of the scan line switchingelement (the scan line selection signal=the output signal of the scanline driver circuit−Vth of the scan line switching element) when anoutput signal of a scan line driver circuit is supplied to a scan linethrough a scan line switching element. A transistor can be used as thescan line switching element, for example; here, the scan line switchingelement is described as a transistor.

The output signal of the scan line driver circuit is inputted to a gateelectrode of the transistor and one of source and drain electrodes,which decrease the amplitude of the output signal of the scan linedriver circuit by the Vth of the transistor (such a signal is a scanline selection signal). For example, the output signal of the scan linedriver circuit is inputted to the gate electrode of the transistor andone of the source and drain electrodes, and the transistor is turned on.Since the transistor is turned on, the potential of the other of thesource and drain electrodes is changed so as to be the same as thepotential of the output signal of the scan line driver circuit. However,a voltage Vgs between the gate electrode and the source electrode of thetransistor sometimes becomes Vth before the potential of the other ofthe source and drain electrodes becomes the same as that of the outputsignal of the scan line driver circuit. In this case, since thetransistor is turned off, the potential of the other of the source anddrain electrodes stops changing, which results in making the amplitudeof a scan line selection signal supplied to the scan line smaller thanthat of the output signal of the scan line driver circuit by of thetransistor.

In another example, a scan line selection signal is sometimes distorted.Further, a rising time and a falling time of the scan line selectionsignal are sometimes long. The above reason brings these phenomena. Forexample, an output signal of the scan line driver circuit is inputted tothe gate electrode of the transistor and one of the source and drainelectrodes and the transistor is turned on. Since the transistor isturned on, the potential of the other of the source and drain electrodesis changed so as to be the same as that of the output signal of the scanline driver circuit. At that time, Vgs of the transistor sometimesbecomes small in accordance with a change of the potential of the otherof the source and drain electrodes of the transistor, so that the scanline selection signal is often distorted and a rising time and a fallingtime often become long.

In order to solve the above problem, a signal with higher amplitude thanan output signal of the scan line driver circuit or a power supplyvoltage is additionally needed but it causes an increase in powerconsumption.

It is an object to provide a semiconductor device which can supply asignal with sufficient amplitude to a scan line while power consumptionis kept small. Further, it is an object to provide a semiconductordevice which can suppress distortion of a signal supplied to the scanline and can make a rising time and a falling time shorten while powerconsumption is kept small.

An embodiment of this invention is a semiconductor device including adisplay element, a plurality of pixels each including at least onetransistor, a scan line driver circuit for supplying a signal forselecting a specific pixel from among the plurality of pixels to a scanline. A pixel electrode layer of the display element, a gate electrodelayer of a transistor, source and drain electrode layers of thetransistor, and a scan line are formed using a light-transmittingconductive layer. The scan line driver circuit includes a transistor anda capacitor for holding voltage between the gate electrode layer and thesource electrode layer of the transistor. The source electrode layer ofthe transistor is connected to the scan line.

An embodiment of this invention is a semiconductor device including adisplay element, a plurality of pixels each including at least one firsttransistor, and a scan line driver circuit supplying a signal forselecting a specific pixel from among the plurality of pixels to a scanline. A pixel electrode layer of the display element, a gate electrodelayer of the first transistor, source and drain electrode layers of thetransistor, and the scan line are formed using a light-transmittingconductive layer. A scan line driver circuit includes a secondtransistor, a capacitor for holding voltage between a gate electrodelayer of the second transistor and a source electrode layer of thesecond transistor, and a third transistor for controlling connectionbetween the gate electrode layer of the second transistor and a groundelectrode. The source electrode of the second transistor is connected tothe scan line.

An embodiment of this invention enables a signal with sufficientamplitude to be supplied to a scan line by bootstrap operation. Further,an embodiment of this invention can suppress distortion of a signal andshorten a rising time and a falling time. Furthermore, an embodiment ofthis invention does not need to have a power supply voltage which ishigher than the voltage of an input signal, which results in low powerconsumption driving.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1F are cross-sectional views illustrating Embodiment 1;

FIGS. 2A and 2C are cross-sectional views and FIGS. 2B-1 and 2B-2 areplan views each illustrating Embodiment 1;

FIGS. 3A-1 and 3A-2 are plan views and FIGS. 3B and 3C arecross-sectional views each illustrating Embodiment 1;

FIGS. 4A to 4E are cross-sectional views illustrating Embodiment 2;

FIGS. 5A to 5E are cross-sectional views illustrating Embodiment 2;

FIG. 6A is a circuit diagram and FIG. 6B is a timing chart eachillustrating Embodiment 3;

FIG. 7A is a circuit diagram and FIG. 7B is a timing chart eachillustrating Embodiment 4;

FIG. 8 is a circuit diagram for illustrating Embodiment 5;

FIG. 9 is a circuit diagram for illustrating Embodiment 6;

FIG. 10 is a circuit diagram for illustrating Embodiment 7;

FIG. 11 is a circuit diagram for illustrating Embodiment 8;

FIG. 12 is a circuit diagram for illustrating Embodiment 9;

FIG. 13 is a timing chart illustrating Embodiment 9;

FIG. 14A is a top view of a liquid crystal display device and FIGS. 14Band 14C are cross-sectional views of the liquid crystal display device;

FIGS. 15A and 15B are cross-sectional views of a light-emitting elementdisplay device;

FIGS. 16A and 16B are cross-sectional views of an electronic paper;

FIGS. 17A to 17H are diagrams each illustrating an example of an actualproduct;

FIGS. 18A to 18H are diagrams each illustrating an example of an actualproduct; and

FIG. 19 is a cross-sectional view illustrating Embodiment 1.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of this invention will be described with reference to thedrawings. Note that this invention is not limited to the followingdescription, and it will be easily understood by those skilled in theart that various changes and modifications can be made without departingfrom the spirit and scope of this invention. Thus, this invention shouldnot be interpreted as being limited to the following description of theembodiments.

Embodiment 1

A semiconductor device and a manufacturing method of the semiconductordevice will be described with reference to FIGS. 1A to 1F and FIGS. 2A,2B-1, 2B-2, and 2C. FIG. 2A illustrates an example of a cross-sectionalstructure of two thin film transistors which have different structuresfrom each other and are formed over the same substrate. FIG. 2Aillustrates a thin film transistor 1470 of a channel-etched type whichis one of bottom gate structures and a thin film transistor 1460 of abottom-contacted type (an inverted coplanar type) which is one of bottomgate structures.

FIG. 2B-1 is a plan view of the channel-etched thin film transistor 1470provided in a driver circuit. FIG. 2A is a cross-sectional view takenalong line C1-C2 in FIG. 2B-1. In addition, FIG. 2C is a cross-sectionalview taken along line C3-C4 in FIG. 2B-1.

The thin film transistor 1470 provided in the driver circuit is achannel-etched thin film transistor and includes a gate electrode layer1401; a first gate insulating layer 1402 a; a second gate insulatinglayer 1402 b; an oxide semiconductor layer including at least a channelformation region 1434, a first high resist drain region 1431, and asecond high resist drain region 1432; a source electrode layer 1405 a;and a drain electrode layer 1405 b over a substrate 1400 having aninsulation surface. Further, an oxide insulating layer 1407 is providedso as to cover the thin film transistor 1470 and to be in contact withthe channel formation region 1434.

The first high resist drain region 1431 is formed in a self-alignedmanner in contact with a bottom surface of the source electrode layer1405 a. Further, the second high resist drain region 1432 is formed in aself-aligned manner in contact with a bottom surface of the drainelectrode layer 1405 b. In addition, the channel formation region 1434is in contact with the oxide insulating layer 1407, has thin thickness,and is a region with higher resist (an I type region) than that of thefirst high resist drain region 1431 and that of the second high resistdrain region 1432.

In addition, in the thin film transistor 1470, it is preferable that ametal material be used for the source electrode layer 1405 a and thedrain electrode layer 1405 b in order to make wirings have lowresistance.

In addition, when a pixel portion and a driver circuit are formed overthe same substrate in the liquid crystal display device, in the drivercircuit, only one of positive polarity and negative polarity is appliedbetween the source and drain electrodes of a thin film transistor forconstituting a logic gate such as an inverter circuit, a NAND circuit, aNOR circuit, and a latch circuit or a thin film transistor forconstituting an analog circuit such as a sense amplifier, a constantvoltage generating circuit, and a VCO. Therefore, the width of thesecond high resist drain region 1432 which needs to withstand voltagemay be designed to be larger than that of the first high resist drainregion 1431. Further, the width of the gate electrode layer overlappingwith the first high resist drain region 1431 and the second high resistdrain region 1432 may be large.

Further, the thin film transistor 1470 provided in the driver circuit isdescribed with use of a single gate thin film transistor, however, amulti gate thin film transistor including a plurality of channelformation regions can be used as necessary.

Further, a conductive layer 1406 is formed over the channel formationregion 1434 to overlap therewith. The conductive layer 1406 iselectrically connected to the gate electrode layer 1401 and has the samepotential as the gate electrode layer 1401, so that a gate voltage canbe applied from the upper and lower sides of the oxide semiconductorprovided between the gate electrode layer 1401 and the conductive layer1406. Further, when the potential of the conductive layer 1406 isdifferent from that of the gate electrode layer 1401 and is, forexample, a fixed potential, GND, and 0 V, the electrical characteristicsof the thin film transistor such as a threshold voltage can becontrolled.

In addition, a protection insulating layer 1408 and a planarizinginsulating layer 1409 are stacked between the conductive layer 1406 andthe oxide insulating layer 1407.

Further, it is preferable to use a structure in which the protectioninsulating layer 1408 is in contact with the first gate insulating layer1402 a provided below the protection insulating layer 1408 or aninsulating layer serving as a base and which prevents an impurity suchas moisture, a hydrogen ion, and OH⁻ from entering the oxidesemiconductor layer from the side direction. In particular, when thefirst gate insulating layer 1402 a or the insulating film serving as abase in contact with the protection insulating layer 1408 is a siliconnitride film, the effect is enhanced.

Note that FIG. 2B-2 is a plan view of the bottom-contacted thin filmtransistor 1460 provided in a pixel. FIG. 2A is a cross-sectional viewtaken along line D1-D2 in FIG. 2B-2. Further, FIG. 2C is across-sectional view taken along line D3-D4 in FIG. 2B-2.

The thin film transistor 1460 provided in the pixel is abottom-contacted thin film transistor and includes a gate electrodelayer 1451, the first gate insulating layer 1402 a, the second gateinsulating layer 1402 b, an oxide semiconductor layer 1454 including achannel formation region, a source electrode layer 1455 a, and a drainelectrode layer 1455 b over the substrate 1400 having an insulationsurface. Further, an oxide insulating layer 1407 is provided so as tocover the thin film transistor 1460 and to be in contact with a topsurface and a side surface of the oxide semiconductor layer 1454.

Note that an AC drive is performed in a liquid crystal display device inorder to prevent deterioration of liquid crystal. The AC drive allowsthe polarity of a signal potential applied to a pixel electrode layer tobe inverted to be negative or positive at regular intervals of time. Ina thin film transistor connected to the pixel electrode layer, a pair ofelectrodes functions alternately as a source electrode layer and a drainelectrode layer respectively. In this specification, one of thin filmtransistors of a pixel is referred to as a source electrode layer andthe other is a drain electrode layer in convenience; actually, in the ACdrive, one of electrodes functions as a source electrode layer and adrain electrode layer, alternately. In addition, in order to reduceleakage current, the width of the gate electrode layer 1451 of the thinfilm transistor 1460 provided in the pixel can be smaller than that ofthe gate electrode layer 1401 of the thin film transistor 1470 of thedriver circuit. In addition, in order to reduce leakage current, thegate electrode layer 1451 of the thin film transistor 1460 provided inthe pixel may be designed not to overlap with the source electrode layer1455 a or the drain electrode layer 1455 b.

Further, the thin film transistor 1460 provided in the pixel isdescribed with use of a single gate thin film transistor; however, amulti gate thin film transistor including a plurality of channelformation regions can be used as necessary.

Further, heat treatment is performed on the oxide semiconductor layer1454 in order to reduce impurities such as moisture (heat treatment fordehydration and dehydrogenation) after at least an oxide semiconductorfilm is formed. After heat treatment for dehydration and dehydrogenationand slow cooling, the oxide insulating layer 1407 is formed in contactwith the oxide semiconductor layer 1454 to reduce the carrierconcentration of the oxide semiconductor layer 1454, which leads toimprovement of the electrical characteristics and reliability of thethin film transistor 1460.

Note that the oxide semiconductor layer 1454 is formed over and partlyoverlaps with the source electrode layer 1455 a and the drain electrodelayer 1455 b. Further, the oxide semiconductor layer 1454 overlaps withthe gate electrode layer 1451 with the first gate insulating layer 1402a and the second gate insulating layer 1402 b therebetween. The channelformation region of the thin film transistor 1460 provided in the pixelis a region where the oxide semiconductor layer 1454 is sandwichedbetween a side surface of the source electrode layer 1455 a and the sidesurface of the drain electrode layer 1455 b which faces the side surfaceof the source electrode layer 1455 a, that is, a region which is incontact with the second gate insulating layer 1402 b and overlaps withthe gate electrode layer 1451.

In addition, in order that a display device of which the aperture ratiois high may be realized using a light-transmitting thin film transistoras the thin film transistor 1460, a light-transmitting conductive filmis used for the source electrode layer 1455 a and the drain electrodelayer 1455 b.

Further, a light-transmitting conductive film is also used for the gateelectrode layer 1451 of the thin film transistor 1460.

Furthermore, in the pixel provided with the thin film transistor 1460, aconductive film having a light-transmitting property with respect tovisible light is used as a pixel electrode layer 1456, the otherelectrode layer (such as a capacitor electrode layer), or the otherwiring layer (such as a capacitor wiring layer); therefore, a displaydevice with a high aperture ratio is realized. Needless to say, it ispreferable that a conductive film having a light-transmitting propertywith respect to visible light also be used for the gate insulating layer1402 a, the gate insulating layer 1402 b, and the oxide insulating layer1407.

In this specification, a film having a light-transmitting property withrespect to visible light is a film with a thickness of whichtransmittance is 75% or more and 100% or less with respect to visiblelight. When the film is conductive, the film is also referred to as atransparent conductive film. In addition, a conductive film which issemi-transmissive with respect to visible light may be used for a gateelectrode layer, a source electrode layer, a drain electrode layer, apixel electrode layer, the other electrode layer, or a metal oxideapplied to the other wiring layer. The words “semi-transmissive withrespect to visible light” means that the transmittance of visible lightis 50% or more and 75% or less.

Manufacturing process of the thin film transistor 1470 and the thin filmtransistor 1460 which are formed over the same substrate is describedbelow with reference to FIGS. 1A to 1F, and FIG. 2A.

First, a light-transmitting conductive film is formed over the substrate1400 having an insulation surface; then, the gate electrode layers 1401and 1451 are formed by a first photolithography process. In addition, ina pixel portion, a capacitor wiring layer is formed by the same firstphotolithography process using a light-transmitting material which isthe same material as the gate electrode layers 1401 and 1451. Further,when the driver circuit needs a capacitor, a capacitor wiring layer isformed not only in the pixel portion but also in the driver circuit.Note that a resist mask may be formed by an ink jet method. When aresist mask may be formed by an ink jet method, a photomask is notneeded; therefore, manufacturing cost can be reduced.

Although there is no particular limitation on a substrate which can beused for the substrate 1400 having an insulation surface, it isnecessary that the substrate have at least enough heat resistance towithstand heat treatment to be performed later. As the substrate 1400having an insulating surface, a barium borosilicate glass substrate, analumino-borosilicate glass substrate, or a glass substrate whosedistortion point is 600° C. to 750° C. can be used.

Note that when heat treatment performed later is performed at hightemperature, it is preferable that a glass substrate whose distortionpoint be 730° C. or more is used as the glass substrate 1400. Further,for example, a glass material such as aluminosilicate glass,aluminoborosilicate glass, or barium borosilicate glass is used for theglass substrate 1400. Note that, in general, a glass substrate containsa larger amount of barium oxide (BaO) than that of boric acid, whereby aheat-resistant glass substrate which is further practical can beobtained. Therefore, a glass substrate containing BaO and B₂O₃ where theamount of BaO is larger than that of B₂O₃ is preferably used.

Note that a substrate formed of an insulator such as a ceramicsubstrate, a quartz substrate, or a sapphire substrate may be usedinstead of the glass substrate 1400. Alternatively, crystallized glassor the like can be used.

Note that an insulating film serving as a base film may be providedbetween the substrate 1400 and the gate electrode layers 1401 and 1451.The base film has a function of preventing diffusion of an impurityelement from the substrate 1400 and can be formed to have a single-layeror stacked-layer structure using one or more of a silicon nitride film,a silicon oxide film, a silicon nitride oxide film, and a siliconoxynitride film.

A conductive material having a light-transmitting property with respectto visible light such as an In—Sn—Zn—O-based metal oxide, anIn—Al—Zn—O-based metal oxide, an Sn—Ga—Zn—O-based metal oxide, anAl—Ga—Zn—O-based metal oxide, an Sn—Al—Zn—O-based metal oxide, anIn—Zn—O-based metal oxide, an Sn—Zn—O-based metal oxide, anAl—Zn—O-based metal oxide, an In—O-based metal oxide, an Sn—O-basedmetal oxide, and a Zn—O-based metal oxide can be used as a material ofthe gate electrode layers 1401 and 1451. The thickness of the gateelectrode layers 1401 and 1451 is appropriately selected in the range of50 nm to 300 nm. As a deposition method of a metal oxide used for thegate electrode layers 1401 and 1451, a sputtering method, a vacuumevaporation method (an electron beam evaporation method), an arc ionplating method, or a spray method is used. Note that when a sputteringmethod is used, deposition is performed using a target including SiO₂ at2 percent by weight or more and 10 percent by weight or less and alight-transmitting conductive film is made to include SiOx (X>0) whichsuppresses crystallization, so that crystallization can be suppressedwhen heat treatment is performed for dehydration and dehydrogenationperformed in a later process.

Next, a gate insulating layer is formed over the gate electrode layers1401 and 1451.

The gate insulating layer can be formed by a single layer or a stackedlayer of a silicon oxide layer, a silicon nitride layer, a siliconoxynitride layer, or a silicon nitride oxide layer and by a plasma CVDmethod or a sputtering method. For example, a silicon oxynitride layermay be formed by a plasma CVD method using a deposition gas containingSiH₄, oxygen, and nitrogen.

In this embodiment, a gate insulating layer is a stacked layer of thefirst gate insulating layer 1402 a with a thickness of 50 nm or more and200 nm or less and the second gate insulating layer 1402 b with athickness of 50 nm or more and 300 nm or less. A silicon nitride film ora silicon nitride oxide film with a thickness of 100 nm is used as thefirst gate insulating layer 1402 a. Further, a silicon oxide film with athickness of 100 nm is used as the second gate insulating layer 1402 b.

Next, after a light-transmitting conductive film is formed over thesecond gate insulating layer 1402 b, the source electrode layer 1455 aand the drain electrode layer 1455 b are formed by a secondphotolithography process (see FIG. 1A). As a deposition method of thelight-transmitting conductive film, a sputtering method, a vacuumevaporation method (an electron beam evaporation method), an arc ionplating method, or a spray method is used. A conductive material havinga light-transmitting property with respect to visible light such as anIn—Sn—Zn—O-based metal oxide, an In—Al—Zn—O-based metal oxide, anSn—Ga—Zn—O-based metal oxide, an Al—Ga—Zn—O-based metal oxide, anSn—Al—Zn—O-based metal oxide, an In—Zn—O-based metal oxide, anSn—Zn—O-based metal oxide, an Al—Zn—O-based metal oxide, an In—O-basedmetal oxide, an Sn—O-based metal oxide, and a Zn—O-based metal oxide canbe used as a material of the conductive film. The thickness of theconductive film is appropriately selected in the range of 50 nm to 300nm. Note that when a sputtering method is used, deposition is performedusing a target including SiO₂ at 2 percent by weight or more and 10percent by weight or less and a light-transmitting conductive film ismade to include SiOx (X>0) which suppresses crystallization, so thatcrystallization can be suppressed when heat treatment is performed fordehydration and dehydrogenation performed in a later process.

Note that a resist mask for forming the source electrode layer 1455 aand the drain electrode layer 1455 b may be formed by an ink jet method.When a resist mask may be formed by an ink jet method, a photomask isnot needed; therefore, manufacturing cost can be reduced.

Next, an oxide semiconductor film with a thickness of 2 nm or more and200 nm or less is formed over the second gate insulating layer 1402 b,the source electrode layer 1455 a, and the drain electrode layer 1455 b.The thickness is preferably 50 nm or less in order that the oxidesemiconductor layer may be amorphous even when heat treatment fordehydration and dehydrogenation is performed after the oxidesemiconductor film is formed. Thin thickness of the oxide semiconductorlayer can suppress crystallization when heat treatment is performedafter the oxide semiconductor layer is formed.

Note that before the oxide semiconductor film is formed by a sputteringmethod, dust on a surface of the second gate insulating layer 1402 b ispreferably removed by reverse sputtering in which an argon gas isintroduced and plasma is generated. The reverse sputtering refers to amethod in which, without application of voltage to a target side, an RFpower source is used for application of voltage to a substrate side inan argon atmosphere to generate plasma in a vicinity of the substrate tomodify a surface. Note that nitrogen, helium, oxygen, or the like may beused instead of an argon atmosphere.

The following film is used for the oxide semiconductor film: anIn—Ga—Zn—O-based non-single-crystal film; an In—Sn—Zn—O-based oxidesemiconductor film, an In—Al—Zn—O-based oxide semiconductor film, aSn—Ga—Zn—O-based oxide semiconductor film, an Al—Ga—Zn—O-based oxidesemiconductor film, a Sn—Al—Zn—O-based oxide semiconductor film, anIn—Zn—O-based oxide semiconductor film, an Sn—Zn—O-based oxidesemiconductor film, an Al—Zn—O-based oxide semiconductor film, anIn—O-based oxide semiconductor film, a Sn—O-based oxide semiconductorfilm, and a Zn—O-based oxide semiconductor film. In this embodiment, theoxide semiconductor film is formed by a sputtering method with use of anIn—Ga—Zn—O-based oxide semiconductor target. Alternatively, the oxidesemiconductor film can be formed by a sputtering method under a rare gas(typically argon) atmosphere, an oxygen atmosphere, or an atmosphereincluding a rare gas (typically argon) and oxygen. Note that when asputtering method is used, deposition is performed using a targetincluding SiO₂ at 2 percent by weight or more and 10 percent by weightor less and the oxide semiconductor film is made to include SiOx (X>0)which suppresses crystallization, so that crystallization can besuppressed when heat treatment is performed for dehydration anddehydrogenation performed in a later process.

Next, the oxide semiconductor film is processed into an island-shapeoxide semiconductor layer by a third photolithography process. Note thatin order to obtain the oxide semiconductor layer overlapping with thesource electrode layer 1455 a and the drain electrode layer 1455 b,materials and conditions of etching are adjusted as appropriate in casethe source electrode layer 1455 a and the drain electrode layer 1455 bshould be removed in etching of the oxide semiconductor layer. Note thata resist mask for forming the island-shape oxide semiconductor layer maybe formed by an ink jet method. When a resist mask may be formed by anink jet method, a photomask is not needed; therefore, manufacturing costcan be reduced.

Next, dehydration and dehydrogenation of the oxide semiconductor layeris performed. Temperature in first heat treatment at which dehydrationand dehydrogenation is performed is 350° C. or more and less than adistortion point of a substrate, or more preferably 400° C. or more.Here, the substrate is introduced into an electric furnace which is oneof heat treatment devices and heat treatment is performed on the oxidesemiconductor layer under a nitrogen atmosphere. Then, reentrance ofwater or hydrogen to the oxide semiconductor is prevented withoutexposure to the air. Thus, oxide semiconductor layers 1403 and 1453 areobtained (see FIG. 1B). In this embodiment, the same furnace is usedfrom heating temperature T at which dehydration and dehydrogenation ofthe oxide semiconductor layer is performed to temperature which isenough to prevent reentrance of water. Specifically, the substrate iscooled slowly until temperature becomes less than heating temperature Tby 100° C. or more under a nitrogen atmosphere. Note that thisembodiment is not limited to a nitrogen atmosphere. Dehydration anddehydrogenation can be performed under helium, neon, argon, or the likeor under reduced pressure.

Note that at the first heat treatment, it is preferable that nitrogen orrare gas such as helium, neon, or argon do not include water, hydrogen,or the like. Alternatively, it is preferable that purity of nitrogen orrare gas such as helium, neon, or argon be 6N (99.9999%) or more, morepreferably 7N (99.99999%) or more (i.e., impurity concentration be 1 ppmor less, more preferably, 0.1 ppm or less).

Further, the oxide semiconductor film is crystallized and can be a microcrystal film or a polycrystalline film depending on a condition of thefirst heat treatment or a material of oxide semiconductor layer.

Further, the first heat treatment of the oxide semiconductor layer canbe performed on the oxide semiconductor film before the oxidesemiconductor film is processed into an island-shape oxide semiconductorlayer. In that case, the substrate is taken out from a heating deviceafter the first heat treatment; then, a photolithography process isperformed.

Furthermore, it is acceptable that heat treatment (heating temperatureis 400° C. or more and less than a distortion point of the substrate) beperformed under an inert gas atmosphere (nitrogen, helium, neon, argon,or the like), an oxygen atmosphere, or reduced pressure beforedeposition of the oxide semiconductor film and the oxide semiconductorlayer be a gate insulating layer in which an impurity such as hydrogenand water are removed.

Next, a metal conductive film is formed over the second gate insulatinglayer 1402 b, a resist mask 1436 is formed by a fourth photolithographyprocess, and etching is selectively performed, so that a metal electrodelayer 1435 is formed (see FIG. 1C). As the material of the metalconductive film, an element selected from Al, Cr, Cu, Ta, Ti, Mo, and W,an alloy containing any of these elements as a component, an alloycontaining these elements in combination, and the like can be used.

As the metal conductive film, it is preferable to use a stacked layer ofthree layers where an aluminum layer are formed over a titanium layerand a titanium layer is formed over the aluminum layer or where analuminum layer is formed over a molybdenum layer and a molybdenum layeris formed over the aluminum layer. Needless to say, a single layer, astacked layer of two layers or a stacked layer of four or more layerscan be used as the metal conductive layer.

Note that in order to selectively remove the metal conductive filmoverlapping with the oxide semiconductor layer 1453, the sourceelectrode layer 1455 a, and the drain electrode layer 1455 b, materialsand conditions of etching are adjusted as appropriate in case the oxidesemiconductor layer 1453, the source electrode layer 1455 a, and thedrain electrode layer 14556 should be removed in etching of the metalconductive film. Note that a resist mask for forming the metal electrodelayer 1435 may be formed by an ink jet method. When a resist mask may beformed by an ink jet method, a photomask is not needed; therefore,manufacturing cost can be reduced.

Next, the resist mask 1436 is removed, a resist mask 1437 is formed by afifth photolithography process, and etching is performed selectively, sothat the source electrode layer 1405 a and the drain electrode layer1405 b are formed (see FIG. 1D). Note that at the fifth photolithographyprocess, only part of the oxide semiconductor layer is etched to form anoxide semiconductor layer 1433 having a groove (depression). Further, aresist mask for forming a groove (depression) in the oxide semiconductorlayer can be formed by an ink jet method. When a resist mask used forforming a groove in the oxide semiconductor layer may be formed by anink jet method, a photomask is not needed; therefore, manufacturing costcan be reduced.

Next, the resist mask 1437 is removed, and the oxide insulating layer1407 is formed as a protection insulating film in contact with a topsurface and a side surface of the oxide semiconductor layer 1453 and thegroove (depression) of the oxide semiconductor layer 1433.

The oxide insulating layer 1407 has a thickness of at least 1 nm or moreand can be formed using a method in which an impurity such as water andhydrogen does not enter the oxide insulating layer 1407 as appropriate,by sputtering method or the like. In this embodiment, a silicon oxidefilm whose thickness is 300 nm is deposited by a sputtering method asthe oxide insulating layer 1407. Temperature of a substrate atdeposition may be room temperature or more and 300° C. or less. In thisembodiment, temperature of a substrate at deposition is 100° C. Thesilicon oxide film can be formed by a sputtering method under a rare gas(typically argon) atmosphere, an oxygen atmosphere, or an atmospherecontaining a rare gas (typically argon) and oxygen. In addition, asilicon oxide target or a silicon target can be used as a target. Forexample, the silicon oxide film can be formed using a silicon target bya sputtering method under an atmosphere including oxygen and nitrogen.As the oxide insulating layer 1407 formed so as to be in contact with alow-resistance oxide semiconductor layer, an inorganic film in which animpurity such as moisture, a hydrogen ion, and OH⁻ is not contained andwhich prevents such an impurity from entering the oxide insulating layerfrom the outside are used; typically, a silicon oxide film, a siliconnitride oxide film, an aluminum oxide film, an aluminum oxynitride film,or the like is used.

Next, second heat treatment (at 200° C. or more and 400° C. or less,preferably; 250° C. or more and 350° C. or less, for example) areperformed under an inert gas atmosphere or an oxygen gas atmosphere (seeFIG. 1E). For example, the second heat treatment is performed under anitrogen atmosphere at 250° C. for one hour. In the second heattreatment, the groove of the oxide semiconductor layer 1433 and the topsurface and the side surface of the oxide semiconductor layer 1453 areheated in contact with the oxide insulating layer 1407.

Through the above process, heat treatment for dehydration anddehydrogenation is performed on the deposited oxide semiconductor filmto lower resistance, and then, a part of the oxide semiconductor film isselectively made to include excessive oxygen. As a result, the channelformation region 1434 overlapping with the gate electrode layer 1401becomes an I type and the first high resist drain region 1431overlapping with the source electrode layer 1405 a and the second highresist drain region 1432 overlapping with the drain electrode layer 1405b are formed in a self-aligned manner. Further, the oxide semiconductorlayer 1453 overlapping with the gate electrode layer 1451 becomes theoxide semiconductor layer 1454 the whole of which is an I type.

Note that the second high resist drain region 1432 (or the first highresist drain region 1431) is formed in the oxide semiconductor layeroverlapped with the drain electrode layer 1405 b (and the sourceelectrode layer 1405 a), so that reliability in forming a driver circuitcan be improved. Specifically, by forming the second high resist drainregion 1432, conductivity can be gradually changed from the drainelectrode layer to the second high resist drain region 1432 and thechannel formation region. Therefore, in the case where a transistor isdriven in the state where the drain electrode layer 1405 b is connectedto a wiring supplying high power supply potential VDD, even when highelectrical field is applied between the gate electrode layer 1401 andthe drain electrode layer 1405 b, the high resist drain region functionsas a buffer and high electric field is not locally applied, so thatwithstand voltage of the transistor can be improved.

In addition, the second high resist drain region 1432 (or the first highresist drain region 1431) is formed in the oxide semiconductor layeroverlapped with the drain electrode layer 1405 b (and the sourceelectrode layer 1405 a), so that leakage current in the channelformation region 1434 in forming the driver circuit can be reduced.

Next, the protection insulating layer 1408 is formed over the oxideinsulating layer 1407 (see FIG. 1F). In this embodiment, a siliconnitride film is formed by an RF sputtering method. An RF sputteringmethod is preferable as a deposition method of the protection insulatinglayer 1408 because of its quantity productivity. As the protectioninsulating layer 1408, an inorganic film in which an impurity such asmoisture, a hydrogen ion, and OFF is not contained and which preventssuch an impurity from entering the oxide insulating layer from theoutside are used: a silicon nitride film, an aluminum nitride film, asilicon nitride oxide film, an aluminum oxynitride film, or the like isused. Needless to say, the protection insulating layer 1408 is alight-transmitting insulating film.

Further, it is preferable to use a structure in which the protectioninsulating layer 1408 is in contact with the first gate insulating layer1402 a provided below the protection insulating layer 1408 or aninsulating layer serving as a base and which prevents an impurity suchas moisture, a hydrogen ion, and OH⁻ from a vicinity of its side fromentering the oxide semiconductor layer. In particular, when the firstgate insulating layer 1402 or the insulating film serving as a base incontact with the protection insulating layer 1408 is a silicon nitridefilm, the effect is enhanced. That is, when a silicon nitride film isprovided over an under surface, a top surface, and a side surface of theoxide semiconductor layer so as to surround the oxide semiconductorlayer, reliability of a display device is improved.

Next, the planarizing insulating layer 1409 is formed over theprotection insulating layer 1408. The planarizing insulating layer 1409can be formed of an organic material having heat resistance, such aspolyimide, acrylic, benzocyclobutene, polyamide, or epoxy. Other thansuch organic materials, it is also possible to use a low-dielectricconstant material (a low-k material), a siloxane-based resin, PSG(phosphosilicate glass), BPSG (borophosphosilicate glass), or the like.Note that the planarizing insulating layer 1409 may be formed bystacking a plurality of insulating films formed of these materials.

Note that the siloxane-based resin corresponds to a resin including aSi—O—Si bond formed using a siloxane-based material as a startingmaterial. The siloxane-based resin may include as a substituent anorganic group (e.g., an alkyl group or an aryl group) or a fluoro group.In addition, the organic group may include a fluoro group.

The formation method of the planarizing insulating layer 1409 is notlimited to a particular method and a method such as a sputtering method,an SOG method, spin coating, dip coating, spray coating, a dropletdischarge method (e.g., an ink jet method, screen printing, or offsetprinting), or the like and a tool such as a doctor knife, a roll coater,a curtain coater, a knife coater, or the like can be used depending onthe material of the planarizing insulating layer.

Next, a resist mask is formed by a sixth photolithography process, andthe planarizing insulating layer 1409, the protection insulating layer1408, and the oxide insulating layer 1407 are etched to form a contacthole which reaches the drain electrode layer 1455 b. In addition,contact holes which reach the gate electrode layers 1401 and 1451 arealso formed. Note that a resist mask for forming a contact hole whichreaches the drain electrode layer 1455 b may be formed by an ink jetmethod. When a resist mask may be formed by an ink jet method, aphotomask is not needed; therefore, manufacturing cost can be reduced.

Next, after the resist mask is removed, a light-transmitting conductivefilm is formed. The light-transmitting conductive film is formed ofindium oxide (In₂O₃), indium oxide-tin oxide alloy (In₂O₃—SnO₂,abbreviated to ITO), or the like by a sputtering method, a vacuumevaporation method, or the like. As for other material of alight-transmitting conductive film, an Al—Zn—O-based non-single-crystalfilm including nitrogen, that is, an Al—Zn—O—N-based non-single-crystalfilm, Zn—O-based non-single-crystal film including nitrogen, or anSn—Zn—O-based non-single-crystal film including nitrogen can be used.Note that the relative proportion (atomic %) of zinc in anAl—Zn—O—N-based non-single-crystal film is 47 atomic % or less, which islarger than the relative proportion (atomic %) of aluminum in thenon-single-crystal film. The relative proportion (atomic %) of aluminumin the non-single-crystal film is larger than that of nitrogen in thenon-single-crystal film. Such a material is etched with a hydrochloricacid-based solution. However, since a residue is easily generatedparticularly in etching ITO, indium oxide-zinc oxide alloy (In₂O₃—ZnO)may be used to improve etching processability.

Note that the unit of the relative proportion in the light-transmittingconductive film is atomic percent, and the relative proportion isevaluated by analysis using an electron probe X-ray microanalyzer(EPMA).

Next, a seventh photolithography process is performed. A resist mask isformed and unnecessary portions are removed by etching, whereby thepixel electrode layer 1456 and the conductive layer 1406 are formed (seeFIG. 2A).

Through the above process, with seven masks, the thin film transistor1470 and the thin film transistor 1460 can be formed over the samesubstrate in the driver circuit and in the pixel portion, respectively.Further, a storage capacitor which is formed using a capacitor wiringlayer and a capacitor electrode layer and which is formed using thefirst gate insulating layer 1402 a and the second gate insulating layer1402 b which serve as a dielectric can be formed over the samesubstrate. The pixel portion is formed by providing the thin filmtransistors 1460 and the storage capacitors for pixels in matrix and adriver circuit including the thin film transistor 1470 is provided in avicinity of the pixel portion, so that one of substrates formanufacturing an active-matrix display device can be formed. In thisspecification, such a substrate is referred to as an active matrixsubstrate for convenience.

Note that the pixel electrode layer 1456 is electrically connected to acapacitor electrode layer through a contact hole formed in theplanarizing insulating layer 1409, the protection insulating layer 1408,and the oxide insulating layer 1407. Note that the capacitor electrodelayer can be formed using the same light-transmitting material and thesame process as the drain electrode layer 1455 b.

The conductive layer 1406 is provided to overlap with the channelformation region 1434 of the oxide semiconductor layer, so that, in abias-temperature stress test (hereinafter, referred to as a BT test) forexamining reliability of a thin film transistor, the amount of change inthreshold voltage of the thin film transistor 1470 between before andafter the BT test can be reduced. Further, the conductive layer 1406 canfunction as a second gate electrode layer. A potential of the conductivelayer 1406 may be the same as or different from that of the gateelectrode layer 1406, or can be GND, 0V, or in a floating state.

Note that in this embodiment, the thin film transistor 1470 for thedriver circuit has the conductive layer 1406 overlapping with thechannel formation region 1434. However, a thin film transistor for thedriver circuit does not need to have the conductive layer 1406. The thinfilm transistor 1470 having the conductive layer 1406 and a thin filmtransistor which does not have the conductive layer 1401 can be formedover the same substrate using the above process.

In a semiconductor device related to an embodiment of this invention,when a gate electrode layer, a source electrode layer, and a drainelectrode layer of a thin film transistor which is used for a pixel, apixel electrode layer of a display element, and a wiring layer such as ascan line and a signal line are formed using a light-transmittingconductive film, the aperture ratio of the pixel can be enhanced. Notethat an oxide semiconductor is not necessarily used for a thin filmtransistor for the driver circuit. Note that when the thin filmtransistor 1470 for the driver circuit is formed over a substrate wherethe thin film transistor 1460 for the pixel is to be formed, as thisembodiment shows, it is preferable to form the thin film transistor 1470together with the thin film transistor 1460 using an oxide semiconductorbecause the number of steps can be reduced. In this case, both the thinfilm transistor 1470 for the driver circuit and the thin film transistor1460 for the pixel are unipolar transistors.

Note that a resist mask for forming the pixel electrode layer 1456 maybe formed by an ink jet method. When a resist mask may be formed by anink jet method, a photomask is not needed; therefore, manufacturing costcan be reduced.

Next, FIG. 19 illustrates an example of a cross-sectional structure ofan active matrix substrate where a capacitor and a thin film transistorare manufactured by the above process.

FIG. 19 illustrates a capacitor 1502 for the pixel and a capacitor 1505for the driver circuit in addition to the thin film transistor 1470 forthe driver circuit and the thin film transistor 1460 for the pixelportion over the same substrate. The capacitor can be manufacturedtogether with the thin film transistor by the above process without anincrease in the number of masks and steps. Further, in a portion whichis to be a display portion of the pixel portion, a scan line, a signalline, and a capacitor wiring layer are formed using a light-transmittingconductive film, which realizes a high aperture ratio. Furthermore, in adriver circuit formed in a region which is not the display portion, ametal wiring can be used in order to lower wiring resistance.

In FIG. 19, the thin film transistor 1470 is a channel-etched thin filmtransistor provided in the driver circuit and the thin film transistor1460 electrically connected to the pixel electrode layer 1456 is abottom-contacted thin film transistor provided in the pixel portion.

A capacitor wiring layer 1500 which is formed using the samelight-transmitting material and formed through the same process as thegate electrode layer 1451 of the thin film transistor 1460 overlaps witha capacitor electrode layer 1501 with the first gate insulating layer1402 a and the second gate insulating layer 1402 b which serve as adielectric and forms the capacitor 1502 of a pixel therebetween. Notethat the capacitor electrode layer 1501 is formed using the samelight-transmitting material and formed through the same process as thesource electrode layer 1455 a or the drain electrode layer 1455 b of thethin film transistor 1460. Accordingly, the thin film transistor 1460and the capacitor 1502 of a pixel has a light-transmitting property, sothat an aperture ratio can be increased.

A light-transmitting property of the capacitor 1502 is important forenhancement of an aperture ratio. In particular, in a small liquidcrystal display panel having a screen size of inch or less, a highaperture ratio can be realized even when a pixel size is miniaturized inorder that high definition of a display image may be achieved by anincrease of the number of scan lines. Further, a light-transmitting filmis used as a component of the thin film transistor 1460 and thecapacitor 1502, whereby a high aperture ratio can be realized even whenone pixel is divided into a plurality of subpixels in order to realize awide viewing angle. That is, an aperture ratio can be large even when adense group of thin film transistors are provided, so that a sufficientarea of a display region can be secured. For example, when one pixelincludes two to four subpixels and the capacitor 1502, since thecapacitor 1502 and the thin film transistor has a light-transmittingproperty, an aperture ratio can be enhanced.

Note that the capacitor 1502 is provided below the pixel electrode layer1456 and the capacitor electrode layer 1501 is electrically connected tothe pixel electrode layer 1456.

In this embodiment, an example of the capacitor 1502 which is formedusing the capacitor electrode layer 1501 and the capacitor wiring layer1500 is illustrated. However, a structure of a capacitor for the pixelis not limited thereto. For example, without a capacitor wiring layer, acapacitor may be formed as follows: a pixel electrode layer overlapswith a scan line of an adjacent pixel with a planarizing insulatinglayer, a protective insulating layer, a first gate insulating layer, anda second gate insulating layer therebetween.

Further, in the case of manufacturing an active-matrix liquid crystaldisplay device, an active-matrix substrate and a counter substrateprovided with a counter electrode are bonded to each other with a liquidcrystal layer therebetween. Note that a common electrode electricallyconnected to the counter electrode on the counter substrate is providedover the active-matrix substrate, and a terminal electrode electricallyconnected to the common electrode is provided in the terminal portion.This terminal electrode is provided so that the common electrode is setto a fixed potential such as GND or 0 V. The terminal electrode can beformed using the same light-transmitting material as the pixel electrodelayer 1456.

Furthermore, a capacitor wiring layer 1503 which is formed using thesame light-transmitting material and formed through the same process asthe gate electrode layer 1401 of the thin film transistor 1470 overlapswith a capacitor electrode layer 1504 with the first gate insulatinglayer 1402 a and the second gate insulating layer 1402 b which serve asa dielectric and forms the capacitor 1505 of a driver circuittherebetween. Note that the capacitor electrode layer 1504 is formedusing the same light-transmitting material and formed through the sameprocess as the source electrode layer 1405 a or the drain electrodelayer 1405 b of the thin film transistor 1470.

Embodiment 2

One embodiment of a semiconductor device and a manufacturing methodthereof will be described with reference to FIGS. 3A-1, 3A-2, 3B, and3C, FIGS. 4A to 4E, and FIGS. 5A to 5E.

FIGS. 3A-1, 3A-2, 3B, and 3C illustrate an example of a plan view and across-sectional view of two thin film transistors which have differentstructures from each other and which are formed over the same substrate.FIGS. 3A-1, 3A-2, 3B, and 3C illustrate a thin film transistor 2410 of achannel-etched type which is one of bottom gate structures and a thinfilm transistor 2420 of a channel-protection type (also referred to as achannel stop type) which is one of bottom gate structures. The thin filmtransistor 2410 and the thin film transistor 2420 can be referred to asinverted staggered thin film transistors.

FIG. 3A-1 is a plan view of the channel-etched thin film transistor 2410provided in a driver circuit. FIG. 3B is a cross-sectional view takenalong line C1-C2 in FIG. 3A-1. FIG. 3C is a cross-sectional view takenalong line C3-C4 in FIG. 3A-1.

The thin film transistor 2410 provided in the driver circuit is achannel-etched thin film transistor and includes a gate electrode layer2411; a first gate insulating layer 2402 a; a second gate insulatinglayer 2402 b; an oxide semiconductor layer 2412 including at least achannel formation region 2413, a first high resist drain region 2414 a,and a second high resist drain region 2414 b; a source electrode layer2415 a; and a drain electrode layer 2415 b over a substrate 2400 havingan insulation surface. Further, an oxide insulating layer 2416 isprovided so as to cover the thin film transistor 2410 and to be incontact with the channel formation region 2413.

The first high resist drain region 2414 a is formed in a self-alignedmanner in contact with a bottom surface of the source electrode layer2415 a. Further, the second high resist drain region 2414 b is formed ina self-aligned manner in contact with a bottom surface of the drainelectrode layer 2415 b. In addition, the channel formation region 2413is in contact with the oxide insulating layer 2416, has thin thickness,and is a region with higher resist (an I type region) than that of thefirst high resist drain region 2414 a and that of the second high resistdrain region 2414 b.

In addition, in the thin film transistor 2410, it is preferable that ametal material be used for the source electrode layer 2415 a and thedrain electrode layer 2415 b in order to make wirings have lowresistance.

In addition, when a pixel portion and a driver circuit are formed overthe same substrate in the liquid crystal display device, in the drivercircuit, only one of positive polarity and negative polarity is appliedbetween the source and drain electrodes of a thin film transistor forconstituting a logic gate such as an inverter circuit, a NAND circuit, aNOR circuit, and a latch circuit or a thin film transistor forconstituting an analog circuit such as a sense amplifier, a constantvoltage generating circuit, and a VCO. Therefore, the width of thesecond high resist drain region 2414 b which needs to withstand voltagemay be designed to be larger than that of the first high resist drainregion 2414 a. Further, the width of the gate electrode layeroverlapping with the first high resist drain region 2414 a and thesecond high resist drain region 2414 b may be large.

Further, the thin film transistor 2410 provided in the driver circuit isdescribed with use of a single gate thin film transistor; however, amulti gate thin film transistor including a plurality of channelformation regions can be used as necessary.

Further, a conductive layer 2417 is formed over the channel formationregion 2413 to overlap therewith. The conductive layer 2417 iselectrically connected to the gate electrode layer 2411 and has the samepotential as the gate electrode layer 2411, so that a gate voltage canbe applied from the upper and lower sides of the oxide semiconductorprovided between the gate electrode layer 2411 and the conductive layer2417. Further, when the potential of the conductive layer 2417 isdifferent from that of the gate electrode layer 2411 and is, forexample, a fixed potential, GND, and 0 V, the electrical characteristicsof the thin film transistor such as a threshold voltage can becontrolled.

In addition, a protection insulating layer 2403 and a planarizinginsulating layer 2404 are stacked between the conductive layer 2417 andthe oxide insulating layer 2416.

Further, it is preferable to use a structure in which the protectioninsulating layer 2403 is in contact with the first gate insulating layer2402 a provided below the protection insulating layer 2403 or aninsulating layer serving as a base and which prevents an impurity suchas moisture, a hydrogen ion, and OH⁻ from entering the oxidesemiconductor layer from the side direction. In particular, when thefirst gate insulating layer 2402 or the insulating film serving as abase in contact with the protection insulating layer 2403 is a siliconnitride film, the effect is enhanced.

Note that FIG. 3A-2 is a plan view of the channel-protective thin filmtransistor 2420 provided in a pixel. FIG. 3B is a cross-sectional viewtaken along line D1-D2 in FIG. 3A-2. Further, FIG. 3C is across-sectional view taken along line D3-D4 in FIG. 3A-2.

The thin film transistor 2420 provided in the pixel is achannel-protective thin film transistor and includes a gate electrodelayer 2421, the first gate insulating layer 2402 a, the second gateinsulating layer 2402 b, an oxide semiconductor layer 2422 including achannel formation region, an oxide insulating layer 2426 which functionsas a channel protection layer, a source electrode layer 2425 a, and adrain electrode layer 2425 b over the substrate 2400 having aninsulation surface. Further, a stacked layer of the protectioninsulating layer 2403 and the planarizing insulating layer 2404 isprovided so as to cover the thin film transistor 2420 and to be incontact with the oxide insulating layer 2426, the source electrode layer2425 a, and the drain electrode layer 24256. The pixel electrode layer2427 which is in contact with the drain electrode layer 2425 b isprovided over the planarizing insulating layer 2404 and is electricallyconnected to the thin film transistor 2420.

Further, heat treatment is performed on the oxide semiconductor layer2422 in order to reduce impurities such as moisture (heat treatment fordehydration and dehydrogenation) after at least an oxide semiconductorfilm are formed. After heat treatment for dehydration anddehydrogenation and slow cooling, the oxide insulating layer 2426 isformed in contact with the oxide semiconductor layer 2422 to reduce thecarrier concentration of the oxide semiconductor layer 2422, which leadsto improvement of the electrical characteristics and reliability of thethin film transistor 2420.

A channel formation region of the thin film transistor 2420 provided inthe pixel is a part of the oxide semiconductor layer 2422. The channelformation region of the thin film transistor 2420 overlaps with the gateelectrode layer 2421 and is in contact with the oxide insulating layer2426 which is a channel protection layer. Since the thin film transistor2420 is protected by the oxide insulating layer 2426, the oxidesemiconductor layer 2422 is prevented from being etched in an etchingprocess where the source electrode layer 2425 a and the drain electrodelayer 2425 b are formed.

In addition, in order that a display device of which the aperture ratiois high may be realized using a light-transmitting thin film transistoras the thin film transistor 2420, a light-transmitting conductive filmis used for the source electrode layer 2425 a and the drain electrodelayer 2425 b.

Further, a light-transmitting conductive film is also used for the gateelectrode layer 2421 of the thin film transistor 2420.

Furthermore, in a pixel provided with the thin film transistor 2420, aconductive film having a light-transmitting property with respect tovisible light is used as the pixel electrode layer 2427, the otherelectrode layer (such as a capacitor electrode layer), or the otherwiring layer (such as a capacitor wiring layer); therefore, a displaydevice with a high aperture ratio is realized. Needless to say, it ispreferable that a conductive film having a light-transmitting propertywith respect to visible light also be used as the gate insulating layer2402 a, the gate insulating layer 2402 b, and the oxide insulating layer2426.

In this specification, a film having a light-transmitting property withrespect to visible light is a film with a thickness of whichtransmittance is 75% or more and 100% or less with respect to visiblelight. When the film is conductive, the film is also referred to as atransparent conductive film. In addition, a conductive film which issemi-transmissive with respect to visible light may be used for a gateelectrode layer, a source electrode layer, a drain electrode layer, apixel electrode layer, the other electrode layer, or a metal oxideapplied to the other wiring layer. The words “semi-transmissive withrespect to visible light” means that the transmittance of visible lightis 50% or more and 75% or less.

Manufacturing process of the thin film transistor 2410 and the thin filmtransistor 2420 which are formed over the same substrate will bedescribed below with reference to FIGS. 4A to 4E and FIGS. 5A to 5E.

First, a light-transmitting conductive film is formed over the substrate2400 having an insulation surface; then, the gate electrode layers 2411and 2421 are formed by the first photolithography process. In addition,in a pixel portion, a capacitor wiring layer is formed by the same firstphotolithography process using a light-transmitting material which isthe same material as the gate electrode layers 2411 and 2421. Further,when the driver circuit needs a capacitor, a capacitor wiring layer isformed not only in the pixel portion but also in the driver circuit.Note that a resist mask may be formed by an ink jet method. When aresist mask may be formed by an ink jet method, a photomask is notneeded; therefore, manufacturing cost can be reduced.

Although there is no particular limitation on a substrate which can beused for the substrate 2400 having an insulation surface, it isnecessary that the substrate have at least enough heat resistance towithstand heat treatment to be performed later. A substrate similar tothe glass substrate used in Embodiment 1 can be used for the substrate2400 having an insulation surface.

Note that a substrate formed of an insulator such as a ceramicsubstrate, a quartz substrate, or a sapphire substrate may be usedinstead of the glass substrate. Alternatively, crystallized glass or thelike can be used.

Note that an insulating film serving as a base film may be providedbetween the substrate 2400 and the gate electrode layers 2411 and 2421.The base film has a function of preventing diffusion of an impurityelement from the substrate 2400 and can be formed to have a single-layeror stacked-layer structure using one or more of a silicon nitride film,a silicon oxide film, a silicon nitride oxide film, and a siliconoxynitride film.

A conductive material having a light-transmitting property with respectto visible light such as an In—Sn—Zn—O-based metal oxide, anIn—Al—Zn—O-based metal oxide, an Sn—Ga—Zn—O-based metal oxide, anAl—Ga—Zn—O-based metal oxide, an Sn—Al—Zn—O-based metal oxide, anIn—Zn—O-based metal oxide, an Sn—Zn—O-based metal oxide, anAl—Zn—O-based metal oxide, an In—O-based metal oxide, an Sn—O-basedmetal oxide, and a Zn—O-based metal oxide can be used as a material ofthe gate electrode layers 2411 and 2421. The thickness of the gateelectrode layers 2411 and 2421 is appropriately selected in the range of50 nm to 300 nm. As a deposition method of a metal oxide used for thegate electrode layers 2411 and 2421, a sputtering method, a vacuumevaporation method (an electron beam evaporation method), an arc ionplating method, or a spray method is used. Note that when a sputteringmethod is used, deposition is performed using a target including SiO₂ at2 percent by weight or more and 10 percent by weight or less and alight-transmitting conductive film is made to include SiOx (X>0) whichsuppresses crystallization, so that crystallization can be suppressedwhen heat treatment is performed for dehydration and dehydrogenationperformed in a later process.

Next, a gate insulating layer is formed over the gate electrode layers2411 and 2421.

The gate insulating layer can be formed by a single-layer or stackedlayers of a silicon oxide layer, a silicon nitride layer, a siliconoxynitride layer, or a silicon nitride oxide layer by a plasma CVDmethod or a sputtering method. For example, a silicon oxynitride layermay be formed using a deposition gas containing SiH₄, oxygen, andnitrogen by a plasma CVD method.

In this embodiment, a gate insulating layer is a stacked layer of thefirst gate insulating layer 2402 a with a thickness of 50 nm or more and200 nm or less and the second gate insulating layer 2402 b with athickness of 50 nm or mom and 300 nm or less. A silicon nitride film ora silicon nitride oxide film with a thickness of 100 nm is used as thefirst gate insulating layer 2402 a. Further, a silicon oxide film with athickness of 100 nm is used as the second gate insulating layer 2402 b.

An oxide semiconductor film 2430 with a thickness of 2 nm or more and200 nm or less is formed over the second gate insulating layer 2402 b.The thickness is preferably 50 nm or less in order that the oxidesemiconductor film may be amorphous even when heat treatment fordehydration and dehydrogenation is performed after formation of theoxide semiconductor film 2430. Thin thickness of the oxide semiconductorfilm can suppress crystallization when heat treatment is performed afterthe oxide semiconductor layer is formed.

Note that before the oxide semiconductor film 2430 is formed by asputtering method, dust on a surface of the second gate insulating layer2402 b is preferably removed by reverse sputtering in which an argon gasis introduced and plasma is generated. The reverse sputtering refers toa method in which, without application of voltage to a target side, anRF power source is used for application of voltage to a substrate sidein an argon atmosphere to generate plasma in a vicinity of the substrateto modify a surface. Note that nitrogen, helium, oxygen, or the like maybe used instead of an argon atmosphere.

The following film is used for the oxide semiconductor film 2430: anIn—Ga—Zn—O-based non-single-crystal film, an In—Sn—Zn—O-based oxidesemiconductor film, an In—Al—Zn—O-based oxide semiconductor film, aSn—Ga—Zn—O-based oxide semiconductor film, an Al—Ga—Zn—O-based oxidesemiconductor film, a Sn—Al—Zn—O-based oxide semiconductor film, anIn—Zn—O-based oxide semiconductor film, an Sn—Zn—O-based oxidesemiconductor film, an Al—Zn—O-based oxide semiconductor film, anIn—O-based oxide semiconductor film, a Sn—O-based oxide semiconductorfilm, and a Zn—O-based oxide semiconductor film. In this embodiment, theoxide semiconductor film is formed by a sputtering method with use of anIn—Ga—Zn—O-based oxide semiconductor target. Alternatively, the oxidesemiconductor film 2430 can be formed by a sputtering method under arare gas (typically argon) atmosphere, an oxygen atmosphere, or anatmosphere including a rare gas (typically argon) and oxygen. Note thatwhen a sputtering method is used, deposition is performed using a targetincluding SiO₂ at 2 percent by weight or more and 10 percent by weightor less and the oxide semiconductor film 2430 is made to include SiOx(X>0) which suppresses crystallization, so that crystallization can besuppressed when heat treatment is performed for dehydration anddehydrogenation performed in a later process.

Next, the oxide semiconductor film 2430 is processed into anisland-shape oxide semiconductor layer by the second photolithographyprocess. Note that a resist mask for forming the island-shape oxidesemiconductor layer may be formed by an ink jet method. When a resistmask may be formed by an ink jet method, a photomask is not needed;therefore, manufacturing cost can be reduced.

Next, dehydration and dehydrogenation of the oxide semiconductor layeris performed. Temperature in a first heat treatment at which dehydrationand dehydrogenation is performed is 350° C. or more and less than adistortion point of a substrate, or more preferably 400° C. or more.Here, the substrate is introduced into an electric furnace which is oneof heat treatment devices and heat treatment is performed on the oxidesemiconductor layer under a nitrogen atmosphere. Then, reentrance ofwater or hydrogen to the oxide semiconductor is prevented withoutexposure to the air. Thus, oxide semiconductor layers 2431 and 2432 areobtained (see FIG. 4B). In this embodiment, the same furnace is usedfrom heating temperature T at which dehydration and dehydrogenation ofthe oxide semiconductor layer is performed to temperature which isenough to prevent reentrance of water under a nitrogen atmosphere.Specifically, the substrate is cooled slowly until temperature becomesless than heating temperature T by 100° C. or more. Note that thisembodiment is not limited to nitrogen atmosphere. Dehydration anddehydrogenation can be performed under helium, neon, argon, or the likeor under reduced pressure.

Note that at the first heat treatment, it is preferable that nitrogen orrare gas such as helium, neon, or argon does not include water,hydrogen, or the like. Alternatively, it is preferable that purity ofnitrogen or rare gas such as helium, neon, or argon be 6N (99.9999%) ormore, more preferably 7N (99.99999%) or more (i.e., impurityconcentration be 1 ppm or less, more preferably, 0.1 ppm or less).

Further, the oxide semiconductor film is crystallized and can be a microcrystal film or a polycrystalline film depending on a condition of thefirst heat treatment or a material of oxide semiconductor layer.

Further, the first heat treatment of the oxide semiconductor film 2430can be performed on the oxide semiconductor film before the oxidesemiconductor film is processed into an island-shape oxide semiconductorlayer. In that case, the substrate is taken out from a heating deviceafter the first heat treatment; then, a photolithography process isperformed.

Furthermore, it is acceptable that heat treatment (heating temperatureis 400° C. or more and less than a distortion point of the substrate) beperformed under an inert gas atmosphere (nitrogen, helium, neon, argon,or the like), an oxygen atmosphere, or reduced pressure beforedeposition of the oxide semiconductor film 2430 and the oxidesemiconductor layer may be a gate insulating layer in which an impuritysuch as hydrogen and moisture is removed.

Next, a metal conductive film is formed over the second gate insulatinglayer 2402 b, the oxide semiconductor layer 2431, and the oxidesemiconductor layer 2432, resist masks 2433 a and 2433 b are formed bythe third photolithography process, and etching is selectivelyperformed, so that metal electrode layers 2434 and 2435 are formed (seeFIG. 4C). As the material of the metal conductive film, an elementselected from Al, Cr, Cu, Ta, Ti, Mo, and W, an alloy containing any ofthese elements as a component, an alloy containing these elements incombination, and the like can be used.

As a metal conductive film, it is preferable to use a stacked layer ofthree layers where an aluminum layer are formed over a titanium layerand a titanium layer are formed over the aluminum layer or where analuminum layer are formed over a molybdenum layer and a molybdenum layerare formed over the aluminum layer. Needless to say, a single layer, astacked layer of two layers or a stacked layer of four or more layerscan be used as the metal conductive layer.

Note that a resist mask for forming the metal electrode layers 2434 and2435 may be formed by an ink jet method. When a resist mask may beformed by an ink jet method, a photomask is not needed; therefore,manufacturing cost can be reduced.

Next, the resist masks 2433 a and 2433 b are removed, resist masks 2436a and 2436 b are formed by the fourth photolithography process, andetching is performed selectively, so that the source electrode layer2415 a and the drain electrode layer 24156 are formed (see FIG. 4D).Note that at the fourth photolithography process, only part of the oxidesemiconductor layer 2431 is etched to form an oxide semiconductor layer2437 having a groove (depression). Further, the resist masks 2436 a and2436 b for forming a groove (depression) in the oxide semiconductorlayer 2431 can be formed by an ink jet method. When a resist mask may beformed by an ink jet method, a photomask is not needed; therefore,manufacturing cost can be reduced.

Next, the resist masks 2436 a and 2436 b are removed, a resist mask 2438covering the oxide semiconductor layer 2437 is formed by the fifthphotolithography process, and the metal electrode layer 2435 over theoxide semiconductor layer 2432 is removed (see FIG. 4E).

Note that, in order to remove the metal conductive layer 2435overlapping with the oxide semiconductor layer 2432 by the fifthphotolithography process, materials and conditions of etching areadjusted as appropriate in case the oxide semiconductor layer 2432should be removed in etching of the metal electrode layer 2435.

The oxide insulating layer 2439 is formed as a protection insulatingfilm in contact with the top surface and the side surface of the oxidesemiconductor layer 2432 and the groove (depression) of the oxidesemiconductor layer 2437.

The oxide insulating layer 2439 has a thickness of at least 1 nm or moreand can be formed using a method in which an impurity such as water andhydrogen does not enter the oxide insulating layer 2439, as appropriate.In this embodiment, a silicon oxide film whose thickness is 300 nm isdeposited by a sputtering method as the oxide insulating layer 2439.Temperature of a substrate at deposition may be room temperature or moreand 300° C. or less. In this embodiment, temperature of a substrate atdeposition is 100° C. The silicon oxide film can be formed by asputtering method under a rare gas (typically argon) atmosphere, anoxygen atmosphere, or an atmosphere containing a rare gas (typicallyargon) and oxygen. In addition, a silicon oxide target or a silicontarget can be used as a target. For example, the silicon oxide film canbe formed using a silicon target by a sputtering method under anatmosphere including oxygen and nitrogen. As the oxide insulating layer2439 formed so as to be in contact with the low-resistance oxidesemiconductor layer, an inorganic film in which an impurity such asmoisture, a hydrogen ion, and OH⁻ is not contained and which preventssuch an impurity from entering the oxide insulating layer from theoutside are used; typically, a silicon oxide film, a silicon nitrideoxide film, an aluminum oxide film, an aluminum oxynitride film, or thelike is used.

Next, second heat treatment (at 200° C. or more and 400° C. or less,preferably; 250° C. or more and 350° C. or less, for example) areperformed under an inert gas atmosphere or an oxygen gas atmosphere (seeFIG. 5A). For example, second heat treatment is performed under anitrogen atmosphere at 250° C. for one hour. In the second heattreatment, the groove of the oxide semiconductor layer 2437 and the topsurface and the side surface of the oxide semiconductor layer 2432 areheated in contact with the oxide insulating layer 2439.

Through the above process, heat treatment for dehydration anddehydrogenation is performed on the deposited oxide semiconductor filmto lower resistance, and then, a part of the oxide semiconductor film isselectively made to include excessive oxygen. As a result, the channelformation region 2413 overlapping with the gate electrode layer 2411becomes an I type and the first high resist drain region 2414 aoverlapping with the source electrode layer 2415 a and the second highresist drain region 2414 b overlapping with the drain electrode layer2415 b are formed in a self-aligned manner. Further, the oxidesemiconductor layer 2432 overlapping with the gate electrode layer 2421becomes the oxide semiconductor layer 2422 when the whole of the oxidesemiconductor layer 2432 overlapping with the gate electrode layer 2421becomes an I type.

However, when heat treatment is performed under a nitrogen atmosphere,an inert gas atmosphere, or reduced pressure while the oxidesemiconductor layer 2422 which is made to have high resist (to be an Itype) is exposed, the resistance of the oxide semiconductor layer 2422which is made to have high resist (to be an I type) is lowered.Therefore, when the oxide semiconductor layer 2422 is exposed, heattreatment is performed under an oxygen gas atmosphere, and N₂O gasatmosphere, or super dry air (of which dew point under air pressure is−40° C. or less, preferably −60° C. or less).

Note that the second high resist drain region 2414 b (or the first highresist drain region 2414 a) is formed in the oxide semiconductor layeroverlapped with the drain electrode layer 2415 b (and the sourceelectrode layer 2415 a), so that reliability in forming the drivercircuit can be improved. Specifically, by forming the second high resistdrain region 2414 b, conductivity can be gradually changed from thedrain electrode layer 2415 b to the second high resist drain region 2414b and the channel formation region 2413. Therefore, in the case where atransistor is driven in the state where the drain electrode layer 2415 bis connected to a wiring supplying high power supply potential VDD, evenwhen high electrical field is applied between the gate electrode layer2411 and the drain electrode layer 2415 b, the high resist drain regionfunctions as a buffer and high electric field is not locally applied, sothat withstand voltage of the transistor can be improved.

In addition, the second high resist drain region 2414 b (or the firsthigh resist drain region 2414 a) is formed in the oxide semiconductorlayer overlapped with the drain electrode layer 2415 b (and the sourceelectrode layer 2415 a), so that leakage current in the channelformation region 2413 in forming the driver circuit can be reduced.

Next, resist masks 2440 a and 2440 b are formed by the sixthphotolithography process, and the oxide insulating layers 2416 and 2426are formed by the oxide insulating layer 2439 selectively etched (seeFIG. 5B). The oxide insulating layer 2426 is provided over a channelformation region of the oxide semiconductor layer 2422 and functions asa channel protection layer. Note that when an oxide insulating layer isused as the gate insulating layer 2402 b as in this embodiment, filmthickness of the oxide insulating layer is sometimes reduced because apart of the gate insulating layer 2402 b is etched by the etchingprocess of the oxide insulating layer 2439. When a nitride insulatingfilm whose selective ratio with respect to the oxide insulating layer2439 is high is used as the gate insulating layer 2402 b, the gateinsulating layer 2402 b is prevented from being partly etched.

Next, after a light-transmitting conductive film is formed over theoxide semiconductor layer 2422 and the oxide insulating layer 2426, thesource electrode layer 2425 a and the drain electrode layer 2425 b areformed by the seventh photolithography process (see FIG. 5C). As adeposition method of the light-transmitting conductive film, asputtering method, a vacuum evaporation method (an electron beamevaporation method), an arc ion plating method, or a spray method isused. A conductive material having a light-transmitting property withrespect to visible light such as an In—Sn—Zn—O-based metal oxide, anIn—Al—Zn—O-based metal oxide, an Sn—Ga—Zn—O-based metal oxide, anAl—Ga—Zn—O-based metal oxide, an Sn—Al—Zn—O-based metal oxide, anIn—Zn—O-based metal oxide, an Sn—Zn—O-based metal oxide, anAl—Zn—O-based metal oxide, an In—O-based metal oxide, an Sn—O-basedmetal oxide, and a Zn—O-based metal oxide can be used as a material ofthe conductive film. The thickness is appropriately selected in therange of 50 nm to 300 nm. Note that when a sputtering method is used,deposition is performed using a target including SiO₂ at 2 percent byweight or more and 10 percent by weight or less and a light-transmittingconductive film is made to include SiOx (X>0) which suppressescrystallization, so that crystallization can be suppressed when heattreatment is performed for dehydration and dehydrogenation performed ina later process.

Note that a resist mask for forming the source electrode layer 2425 aand the drain electrode layer 2425 b may be formed by an ink jet method.When a resist mask may be formed by an ink jet method, a photomask isnot needed; therefore, manufacturing cost can be reduced.

Next, the protection insulating layer 2403 is formed over the oxideinsulating layer 2416, the oxide insulating layer 2426, the sourceelectrode layer 2425 a, and the drain electrode layer 2425 b. In thisembodiment, a silicon nitride film is formed by an RF sputtering method.An RF sputtering method is preferable as a deposition method of theprotection insulating layer 2403 because of its quantity productivity.As the protection insulating layer 2403, an inorganic film in which animpurity such as moisture, a hydrogen ion, an oxygen ion, and OH⁻ is notcontained and which prevents such an impurity from entering the oxideinsulating layer from the outside are used: a silicon oxide film, asilicon nitride oxide film, an aluminum nitride film, an aluminumoxynitride film, or the like is used. Needless to say, the protectioninsulating layer 2403 is a light-transmitting insulating film.

Further, it is preferable to use a structure in which the protectioninsulating layer 2403 is in contact with the first gate insulating layer2402 a provided below the protection insulating layer 2403 or aninsulating layer serving as a base and which prevents an impurity suchas moisture, a hydrogen ion, and OH⁻ from a vicinity of its side fromentering the oxide semiconductor layer. In particular, when the firstgate insulating layer 2402 or the insulating film serving as a base incontact with the protection insulating layer 2403 is a silicon nitridefilm, the effect is enhanced. That is, when a silicon nitride film isprovided over an under surface, a top surface, and a side surface of theoxide semiconductor layer so as to surround the oxide semiconductorlayer, reliability of a display device is improved.

Next, the planarizing insulating layer 2404 is formed over theprotection insulating layer 2403. The planarizing insulating layer 2404can be formed of an organic material having heat resistance, such aspolyimide, acrylic, benzocyclobutene, polyamide, or epoxy. Other thansuch organic materials, it is also possible to use a low-dielectricconstant material (a low-k material), a siloxane-based resin, PSG(phosphosilicate glass), BPSG (borophosphosilicate glass), or the like.Note that the planarizing insulating layer 2404 may be formed bystacking a plurality of insulating films formed of these materials.

Note that the siloxane-based resin corresponds to a resin including aSi—O—Si bond formed using a siloxane-based material as a startingmaterial. The siloxane-based resin may include as a substituent anorganic group (e.g., an alkyl group or an aryl group) or a fluoro group.In addition, the organic group may include a fluoro group.

The formation method of the planarizing insulating layer 2404 is notlimited to a particular method and a method such as a sputtering method,an SOG method, spin coating, dip coating, spray coating, a dropletdischarge method (e.g., an inkjet method, screen printing, or offsetprinting), or the like and a tool such as a doctor knife, a roll coater,a curtain coater, a knife coater, or the like can be used depending onthe material of the planarizing insulating layer.

Next, a resist mask is formed by an eighth photolithography process, andthe planarizing insulating layer 2404 and the protection insulatinglayer 2403 are etched to form a contact hole 2441 which reaches thedrain electrode layer 2425 b (see FIG. 5D). In addition, contact holeswhich reach the gate electrode layers 2411 and 2421 are also formed.Note that a resist mask for forming the contact hole which reaches thedrain electrode layer 2425 b may be formed by an ink jet method. When aresist mask may be formed by an ink jet method, a photomask is notneeded; therefore, manufacturing cost can be reduced.

Next, after the resist mask is removed, a light-transmitting conductivefilm is formed. The light-transmitting conductive film is formed ofindium oxide (In₂O₃), indium oxide-tin oxide alloy (In₂O₃—SnO₂,abbreviated to ITO), or the like by a sputtering method, a vacuumevaporation method, or the like. As for other material of alight-transmitting conductive film, an Al—Zn—O-based non-single-crystalfilm including nitrogen, that is, an Al—Zn—O—N-based non-single-crystalfilm, a Zn—O-based non-single-crystal film including nitrogen, or anSn—Zn—O-based non-single-crystal film including nitrogen can be used.Note that the relative proportion (atomic %) of zinc in anAl—Zn—O—N-based non-single-crystal film is 47 atomic % or less, which islarger than the relative proportion (atomic %) of aluminum in thenon-single-crystal film. The relative proportion (atomic %) of aluminumin the non-single-crystal film is larger than that of nitrogen in thenon-single-crystal film. Such a material is etched with a hydrochloricacid-based solution. However, since a residue is easily generatedparticularly in etching ITO, indium oxide-zinc oxide alloy (In₂O₃—ZnO)may be used to improve etching processability.

Note that the unit of the relative proportion in the light-transmittingconductive film is atomic percent, and the relative proportion isevaluated by analysis using an electron probe X-ray microanalyzer(EPMA).

Next, a ninth photolithography process is performed. A resist mask isformed, and unnecessary portions are removed by etching, whereby thepixel electrode layer 2427 and the conductive layer 2417 are formed (seeFIG. 5E).

Through the above process, with nine masks, the thin film transistor2410 and the thin film transistor 2420 can be formed over the samesubstrate in the driver circuit and in the pixel portion, respectively.The thin film transistor 2410 for the driver circuit is a channel-etchedthin film transistor including the oxide semiconductor layer 2412 havingthe first high resist drain region 2414 a, the second high resist drainregion 2414 b, and the channel formation region 2413. The thin filmtransistor 2420 for the pixel is a channel-protective thin filmtransistor having the oxide semiconductor layer 2422 whole of whichbecomes an I type.

Further, a storage capacitor which is formed using a capacitor wiringlayer and a capacitor electrode layer and which is formed using thefirst gate insulating layer 2402 a and the second gate insulating layer2402 b which serve as a dielectric can be formed over the samesubstrate. The pixel portion is formed by providing the thin filmtransistors 2420 and the storage capacitors for pixels in matrix and adriver circuit including the thin film transistor 2410 is provided in avicinity of the pixel portion, so that one of substrates formanufacturing an active-matrix display device can be formed. In thisspecification, such a substrate is referred to as an active matrixsubstrate for convenience.

Note that the pixel electrode layer 2427 is electrically connected to acapacitor electrode layer through a contact hole formed in theplanarizing insulating layer 2404 and the protection insulating layer2403. Note that the capacitor electrode layer can be formed using thesame light-transmitting material and the same process as the sourceelectrode layer 2425 a and the drain electrode layer 2425 b.

The conductive layer 2417 is provided to overlap with the channelformation region 2413 of the oxide semiconductor layer 2412, so that, ina bias-temperature stress test (hereinafter, referred to as a BT test)for examining reliability of a thin film transistor, the amount ofchange in threshold voltage of the thin film transistor 2410 betweenbefore and after the BT test can be reduced. Further, the conductivelayer 2417 can function as a second gate electrode layer. A potential ofthe conductive layer 2417 may be the same as or different from that ofthe gate electrode layer 2411, or can be GND, 0V, or in a floatingstate.

Note that in this embodiment, the thin film transistor 2410 for thedriver circuit has the conductive layer 2417 overlapping with thechannel formation region 2413. However, a thin film transistor for thedriver circuit does not need to have the conductive layer 2417. The thinfilm transistor 2410 having the conductive layer 2417 and a thin filmtransistor which does not have the conductive layer 2417 can be formedover the same substrate using the above process.

Further, an oxide semiconductor is not necessarily used for a thin filmtransistor for the driver circuit. Note that when the thin filmtransistor 2410 for the driver circuit is formed over a substrate wherethe thin film transistor 2420 for the pixel is to be formed, as thisembodiment shows, it is preferable to form the thin film transistor 2410together with the thin film transistor 2420 using an oxide semiconductorbecause the number of steps can be reduced. In this case, both the thinfilm transistor 2410 for the driver circuit and the thin film transistor2420 for the pixel are unipolar transistors.

Note that a resist mask for forming the pixel electrode layer 2427 maybe formed by an ink jet method. When a resist mask may be formed by anink jet method, a photomask is not needed; therefore, manufacturing costcan be reduced.

Embodiment 3

This embodiment describes an example of a semiconductor device in whicha plurality of signals can be obtained from one signal. Here, the casewhere three signals can be obtained from one signal is described forexample, this embodiment is not limited thereto. A various cases areacceptable as long as two or more signals can be obtained from onesignal.

First, a structure of the semiconductor device of this embodiment willbe described with reference to FIG. 6A.

A circuit 100 includes a circuit 110, a circuit 120, and a circuit 130.The circuit 110 includes a transistor 111 corresponding to a scan lineswitching element, a circuit 112, and a capacitor 114. The circuit 120includes a transistor 121 corresponding to a scan line switchingelement, a circuit 122, and a capacitor 124. The circuit 130 includes atransistor 131 corresponding to a scan line switching element, a circuit132, and a capacitor 134. A signal IN, a signal CK1, a signal CK2, asignal CK3, a signal OUT1, a signal OUT2, and signal OUT3 aretransmitted through a wiring 140, a wiring 141, a wiring 142, a wiring143, a wiring 151, a wiring 152, and a wiring 153, respectively.

Next, a connection relation will be described.

The circuit 100 is connected to the wiring 140, the wiring 141, thewiring 142, and the wiring 143. The circuit 110 is connected to thewiring 140, the wiring 141, and the wiring 151. The circuit 120 isconnected to the wiring 140, the wiring 142, and the wiring 152. Thecircuit 130 is connected to the wiring 140, the wiring 143, and thewiring 153. That is, the wiring 140 is connected to each of the circuit110, the circuit 120, and the circuit 130.

Next, operation of the semiconductor device in FIG. 6A is described withreference to a timing chart in FIG. 6B.

The timing chart in FIG. 6B has a period T1, a period T2, and a periodT3. The signal IN is an input signal of the circuit 100. The signal CK1,the signal CK2, and the signal CK3 are input signals to the circuit 110,the circuit 120, and the circuit 130, respectively. The signal OUT1, thesignal OUT2, and the signal OUT3 are output signals from the circuit110, the circuit 120, and the circuit 130, respectively.

First, operation of a semiconductor device in FIG. 6A in the period T1is described. In the period T1, the signal IN is in an H level, thesignal CK1 is in the H level, the signal CK2 is in an L level, and thesignal CK3 is in the L level. Then, the transistor 111 included in thecircuit 110 is turned on; therefore, the signal OUT1 is in the H level,the signal OUT2 is in the L level, and the signal OUT3 is in the Llevel.

Next, in the period T2, the signal IN is in the H level, the signal CK1is in the L level, the signal CK2 is in the H level, and the signal CK3is in the L level. Then, the transistor 121 included in the circuit 120is turned on; therefore, the signal OUT1 is in the L level, the signalOUT2 is in the H level, and the signal OUT3 is in the L level.

Then, in the period T3, the signal IN is in the H level, the signal CK1is in the L level, the signal CK2 is in the L level, and the signal CK3is in the H level. Then, the transistor 131 included in the circuit 130is turned on; therefore, the signal OUT1 is in the L level, the signalOUT2 is in the L level, and the signal OUT3 is in the H level.

Then, the signal OUT1, the signal OUT2, and the signal OUT3 outputtedfrom the circuit 110, the circuit 120, and the circuit 130,respectively, are inputted as scan line selection signals from a scanline driver circuit to corresponding scan lines.

In this manner, a plurality of signals can be obtained from the signalIN. At this time, capacitive coupling of the capacitor 114, thecapacitor 124, and the capacitor 134 included in the circuit 110, thecircuit 120, and the circuit 130, respectively, makes the amplitude ofthe signal OUT1, the signal OUT2, and the signal OUT3 the same as thatof the signal CK1, the signal CK2, and the signal CK3, respectively.

Further, each of potentials of respective gate electrodes of thetransistor 111, the transistor 121, and the transistor 131 is increasedby bootstrap operation in accordance with the respective signals OUT1,OUT2, and OUT3. That is, Vgs of a transistor can be increased or keptlarge, so that distortion of the signals OUT1 to OUT3 is decreased.Alternatively, rising times or falling times of the signals OUT1 to OUT3can be shortened.

In addition, since a signal which has larger amplitude than the signalIN or a power supply voltage is not additionally needed, powerconsumption can be reduced.

Embodiment 4

In this embodiment, a specific example of Embodiment 3 is described.

First, a structure of a semiconductor device of this embodiment will bedescribed with reference to FIG. 7A.

The circuit 100 includes the circuit 110, the circuit 120, and thecircuit 130. The circuit 110 includes the transistor 111, a transistor113, a transistor 115, and the capacitor 114. The circuit 120 includesthe transistor 121, a transistor 123, a transistor 125, and thecapacitor 124. The circuit 130 includes the transistor 131, a transistor133, a transistor 135, and the capacitor 134. A signal IN1, a signalCK1, a signal CK2, a signal CK3, a signal IN2, a signal OUT1, a signalOUT2, and signal OUT3 are transmitted through the wiring 140, the wiring141, a wiring 142, the wiring 143, a wiring 240, the wiring 151, thewiring 152, and the wiring 153, respectively.

Next, a connection relation will be described.

The circuit 100 is connected to the wiring 140, the wiring 141, thewiring 142, the wiring 143, and the wiring 240. The circuit 110 isconnected to the wiring 140, the wiring 141, the wiring 240, and thewiring 151. The circuit 120 is connected to the wiring 140, the wiring142, the wiring 240, and the wiring 152. The circuit 130 is connected tothe wiring 140, the wiring 143, the wiring 240, and the wiring 153. Thatis, the wiring 140 and the wiring 240 are connected to each of thecircuit 110, the circuit 120, and the circuit 130.

A gate electrode of the transistor 111 included in the circuit 110 isconnected to one of source and drain electrodes of the transistor 115and one of electrodes of the capacitor 114. One of source and drainelectrodes of the transistor 111 is connected to the wiring 141. Theother of the source and drain electrodes of the transistor 111 isconnected to the wiring 151 and the other of the electrodes of thecapacitor 114. A gate electrode of the transistor 115 is connected tothe wiring 140 and the other of the source and drain electrodes of thetransistor 115. One of the source and drain electrodes of the transistor115 is connected to the gate electrode of the transistor 111 and the oneof the electrodes of the capacitor 114. A gate electrode of thetransistor 113 is connected to the wiring 240. One of source and drainelectrodes of the transistor 113 is connected to the gate electrode ofthe transistor 111, the one of the source and drain electrodes of thetransistor 115, and the one of the electrodes of the capacitor 114. Theother of the source and drain electrodes of the transistor 113 isconnected to a GND (ground) electrode.

A gate electrode of the transistor 121 included in the circuit 120 isconnected to one of source and drain electrodes of the transistor 125and one of electrodes of the capacitor 124. One of source and drainelectrodes of the transistor 121 is connected to the wiring 142. Theother of the source and drain electrodes of the transistor 121 isconnected to the wiring 152 and the other of electrodes of the capacitor124. A gate electrode of the transistor 125 is connected to the wiring140 and the other of the source and drain electrodes of the transistor125. The one of the source and drain electrodes of the transistor 125 isconnected to the gate electrode of the transistor 121 and the one of theelectrodes of the capacitor 124. A gate electrode of the transistor 123is connected to the wiring 240. One of the source and drain electrodesof the transistor 123 is connected to the gate electrode of thetransistor 121, the one of the source and drain electrodes of thetransistor 125, and the one of the electrodes of the capacitor 124. Theother of the source and drain electrodes of the transistor 123 isconnected to the GND (ground) electrode.

A gate electrode of the transistor 131 included in the circuit 130 isconnected to one of source and drain electrodes of the transistor 135and one of electrodes of the capacitor 134. One of source and drainelectrodes of the transistor 131 is connected to the wiring 143. Theother of the source and drain electrodes of the transistor 131 isconnected to the wiring 153 and the other of the electrodes of thecapacitor 134. A gate electrode of the transistor 135 is connected tothe wiring 140 and the other of the source and drain electrodes of thetransistor 135. The one of the source and drain electrodes of thetransistor 135 is connected to the gate electrode of the transistor 131and the one of the electrodes of the capacitor 134. A gate electrode ofthe transistor 133 is connected to the wiring 240. One of source anddrain electrodes of the transistor 133 is connected to the gateelectrode of the transistor 131, the one of the source and drainelectrodes of the transistor 135, and the one of the electrodes of thecapacitor 134. The other of the source and drain electrodes of thetransistor 133 is connected to the GND (ground) electrode.

Next, operation of the semiconductor device in FIG. 7A is described withreference to a timing chart in FIG. 7B.

The timing chart in FIG. 7B has a period T1, a period T2, a period T3, aperiod T4, a period T5, and a period T6. The signal IN1 is an inputsignal of the circuit 100 in a first stage. The signal IN2 is an inputsignal of the circuit 100 in a second stage. The signal CK1, the signalCK2, and the signal CK3 are input signals of the circuits 110, thecircuits 120, and the circuits 130, respectively, included in thecircuits 100 in the first stage and the second stage. The signal OUT1,the signal OUT2, and the signal OUT3 are output signals from thecircuits 110, the circuits 120, and the circuits 130, respectively,included in the circuit 100 in the first stage. The signal OUT1, thesignal OUT2, and the signal OUT3 are inputted as scan line selectionsignals from a scan line driver circuit to corresponding scan lines.

First, operation of a semiconductor device in FIG. 7A in the period T1is described. In the period T1, the signal IN1 is in an H level, thesignal IN2 is in an L level, the signal CK1 is in the H level, thesignal CK2 is in the L level, and the signal CK3 is in the L level.Then, the transistor 111 included in the circuit 110 is turned on;therefore, the signal OUT1 is in the H level, the signal OUT2 is in theL level, and the signal OUT3 is in the L level.

Next, in the period T2, the signal IN1 is in the H level, the signal IN2is in the L level, the signal CK1 is in the L level, the signal CK2 isin the H level, and the signal CK3 is in the L level. Then, thetransistor 121 included in the circuit 120 is turned on; therefore, thesignal OUT1 is in the L level, the signal OUT2 is in the H level, andthe signal OUT3 is in the L level. At that time, the transistor 111included in the circuit 110 is kept on.

Then, in the period T3, the signal IN1 is in the H level, the signal IN2is in the L level, the signal CK1 is in the L level, the signal CK2 isin the L level, and the signal CK3 is in the H level. Then, thetransistor 131 included in the circuit 130 is turned on; therefore, thesignal OUT1 is in the L level, the signal OUT2 is in the L level, andthe signal OUT3 is in the H level. At that time, the transistor 111included in the circuit 110 and the transistor 121 included in thecircuit 120 are kept on.

That is, in the period T3, the transistor 111, the transistor 121, andthe transistor 131 are kept on. If the state is continued, the signalOUT1, the signal OUT2, and the signal OUT3 go into the H level when thesignal CK1, the signal CK2, and the signal CK3 go into the H level afterthe period T3 is over, which sometimes cause a defect in scan lineselection.

Next, in the period T4, the signal IN1 is in the L level, the signal IN2is in the H level, the signal CK1 is in the H level, the signal CK2 isin the L level, and the signal CK3 is in the L level. When the signalIN2 goes into the H level, the transistor 113 included in the circuit110, the transistor 123 included in the circuit 120, and the transistor133 included in the circuit 130 is turned on. The other of drain andsource electrodes of each of these transistors is connected to the GNDelectrode, so that the potential of one of source and drain electrodesof each of these transistors goes into the L level. Therefore, the gateelectrodes of the transistor 111, the transistor 121, and the transistor131 connected to the ones of the source and drain electrodes of thetransistors 113, 123, 133, respectively go into the L level, whereby thetransistors 111, 121, and 131 are turned off. Thus, in the period T4,the signal OUT1 can be kept in the L level even when the signal CK1 isin the H level.

In the period T5 and the period T6, as in the period T4, in the casewhere the signal IN2 is in the H level, since the transistor 111, thetransistor 121, and the transistor 131 are off, the signal OUT2 and thesignal OUT3 can be kept in the L level even when the signal CK2 and thesignal CK3 are in the H level. In addition, at that time, the signalOUT4, the signal OUT5, and the signal OUT6 outputted from the circuit100 in the second stage sequentially go into the H level as in the casewhere the signal IN1 are inputted to the circuit 100 in the first stage.

In the case where a structure where the transistor 111, the transistor121, and the transistor 131 are not turned off is used, the signal OUT1,the signal OUT2, and the signal OUT3 go into the H level at the sametime as the signal CK1, the signal CK2, and the signal CK3 go into the Hlevel, which sometimes cause a defect in scan line selection.

Embodiment 5

In this embodiment, another specific example of Embodiment 3 isdescribed.

First, a structure of a semiconductor device of this embodiment will bedescribed with reference to FIG. 8.

The circuit 100 includes the circuit 110, the circuit 120, and thecircuit 130. The circuit 110 includes the transistor 111, the transistor113, the transistor 115, a transistor 116, and the capacitor 114. Thecircuit 120 includes the transistor 121, the transistor 123, thetransistor 125, a transistor 126, and the capacitor 124. The circuit 130includes the transistor 131, the transistor 133, the transistor 135, atransistor 136, and the capacitor 134. A signal IN1, a signal CK1, asignal CK2, a signal CK3, a signal IN2, a signal OUT1, a signal OUT2,and signal OUT3 are transmitted through the wiring 140, the wiring 141,the wiring 142, the wiring 143, a wiring 240, the wiring 151, the wiring152, and the wiring 153, respectively.

Next, a connection relation will be described.

The circuit 100 is connected to the wiring 140, the wiring 141, thewiring 142, the wiring 143, and the wiring 240. The circuit 110 isconnected to the wiring 140, the wiring 141, the wiring 240, and thewiring 151. The circuit 120 is connected to the wiring 140, the wiring142, the wiring 240, and the wiring 152. The circuit 130 is connected tothe wiring 140, the wiring 143, the wiring 240, and the wiring 153. Thatis, the wiring 140 and the wiring 240 are connected to each of thecircuit 110, the circuit 120, and the circuit 130.

The gate electrode of the transistor 111 included in the circuit 110 isconnected to one of the source and drain electrodes of the transistor115 and one of the electrodes of the capacitor 114. One of the sourceand drain electrodes of the transistor 111 is connected to the wiring141. The other of the source and drain electrodes of the transistor 111is connected to the wiring 151, one of the source and drain electrodesof the transistor 116, and the other of the electrodes of the capacitor114. The gate electrode of the transistor 115 is connected to the wiring140 and the other of the source and drain electrodes of the transistor115. One of the source and drain electrodes of the transistor 115 isconnected to the gate electrode of the transistor 111 and the one of theelectrodes of the capacitor 114. The gate electrode of the transistor113 is connected to the wiring 240. One of the source and drainelectrodes of the transistor 113 is connected to the gate electrode ofthe transistor 111, one of the source and drain electrodes of thetransistor 115, and the one of the electrodes of the capacitor 114. Theother of the source and drain electrodes of the transistor 113 isconnected to the GND (ground) electrode. A gate electrode of thetransistor 116 is connected to the wiring 143. The one of the source anddrain electrodes of the transistor 116 is connected to the wiring 151,the other of the source and drain electrodes of the transistor 111, andthe other of the electrodes of the capacitor 114. The other of thesource and drain electrodes of the transistor 116 is connected to theGND (ground) electrode.

A gate electrode of the transistor 121 included in the circuit 120 isconnected to one of the source and drain electrodes of the transistor125 and one of the electrodes of the capacitor 124. One of source anddrain electrodes of the transistor 121 is connected to the wiring 142.The other of the source and drain electrodes of the transistor 121 isconnected to the wiring 152, one of the source and drain electrodes ofthe transistor 126, and the other of electrodes of the capacitor 124. Agate electrode of the transistor 125 is connected to the wiring 140 andthe other of the source and drain electrodes of the transistor 125. Oneof the source and drain electrodes of the transistor 125 is connected tothe gate electrode of the transistor 121 and the one of the electrodesof the capacitor 124. A gate electrode of the transistor 123 isconnected to the wiring 240. One of the source and drain electrodes ofthe transistor 123 is connected to the gate electrode of the transistor121, the one of the source and drain electrodes of the transistor 125,and the one of the electrodes of the capacitor 124. The other of thesource and drain electrodes of the transistor 123 is connected to theGND (ground) electrode. A gate electrode of the transistor 126 isconnected to the wiring 141. The one of the source and drain electrodesof the transistor 126 is connected to the wiring 152, the other of thesource and drain electrodes of the transistor 121, and the one of theelectrodes of the capacitor 124. The other of the source and drainelectrodes of the transistor 126 is connected to the GND (ground)electrode.

The gate electrode of the transistor 131 included in the circuit 130 isconnected to one of the source and drain electrodes of the transistor135 and one of electrodes of the capacitor 134. One of the source anddrain electrodes of the transistor 131 is connected to the wiring 143.The other of the source and drain electrodes of the transistor 131 isconnected to the wiring 153, one of source and drain electrodes of thetransistor 136, and the other of the electrodes of the capacitor 134.The gate electrode of the transistor 135 is connected to the wiring 140and the other of the source and drain electrodes of the transistor 135.The one of the source and drain electrodes of the transistor 135 isconnected to the gate electrode of the transistor 131 and the one of theelectrodes of the capacitor 134. The gate electrode of the transistor133 is connected to the wiring 240. One of the source and drainelectrodes of the transistor 133 is connected to the gate electrode ofthe transistor 131, the one of the source and drain electrodes of thetransistor 135, and the one of the electrodes of the capacitor 134. Theother of the source and drain electrodes of the transistor 133 isconnected to the GND (ground) electrode. A gate electrode of thetransistor 136 is connected to the wiring 142. One of the source anddrain electrodes of the transistor 136 is connected to the wiring 153,one of the source and drain electrodes of the transistor 131, and one ofelectrodes of the capacitor 134. The other of the source and drainelectrodes of the transistor 136 is connected to the GND (ground)electrode.

Next, operation of the semiconductor device in FIG. 8 is described withreference to the timing chart in FIG. 7B.

The timing chart in FIG. 7B has the period T1, the period T2, the periodT3, the period T4, the period T5, and the period T6. The signal IN1 isan input signal of the circuit 100 in a first stage. The signal IN2 isan input signal of the circuit 100 in a second stage. The signal CK1,the signal CK2, and the signal CK3 are input signals of the circuits110, the circuits 120, and the circuits 130, respectively, included inthe circuits 100 in the first stage and the second stage. The signalOUT1, the signal OUT2, and the signal OUT3 are output signals from thecircuits 110, the circuits 120, and the circuits 130, respectively,included in the circuit 100 in the first stage. Then, the signal OUT1,the signal OUT2, and the signal OUT3 are inputted as scan line selectionsignals from a scan line driver circuit to corresponding scan lines.

First, operation of a semiconductor device in FIG. 8 in the period T1 isdescribed. In the period T1, the signal IN1 is in the H level, thesignal IN2 is in the L level, the signal CK1 is in the H level, thesignal CK2 is in the L level, and the signal CK3 is in the L level.Then, the transistor 111 included in the circuit 110 is turned on;therefore, the signal OUT1 is in the H level, the signal OUT2 is in theL level, and the signal OUT3 is in the L level. At that time, thetransistor 126 included in the circuit 120 is turned on and the signalOUT2 goes into the L level.

Next, in the period T2, the signal IN1 is in the H level, the signal IN2is in the L level, the signal CK1 is in the L level, the signal CK2 isin the H level, and the signal CK3 is in the L level. Then, thetransistor 121 included in the circuit 120 is turned on; therefore, thesignal OUT1 is in the L level, the signal OUT2 is in the H level, andthe signal OUT3 is in the L level. At that time, the transistor 111included in the circuit 110 is kept on. Further, the transistor 136included in the circuit 130 is turned on and the signal OUT3 goes intothe L level.

Then, in the period T3, the signal IN1 is in the H level, the signal IN2is in the L level, the signal CK1 is in the L level, the signal CK2 isin the L level, and the signal CK3 is in the H level. Then, thetransistor 131 included in the circuit 130 is turned on; therefore, thesignal OUT1 is in the L level, the signal OUT2 is in the L level, andthe signal OUT3 is in the H level. At that time, the transistor 111included in the circuit 110 and the transistor 121 included in thecircuit 120 are kept on. Further, the transistor 116 included in thecircuit 110 is turned on and the signal OUT1 goes into the L level.

Next, in the period T4, the signal IN1 is in the L level, the signal IN2is in the H level, the signal CK1 is in the H level, the signal CK2 isin the L level, and the signal CK3 is in the L level. When the signalIN2 goes into the H level, the transistor 113 included in the circuit110, the transistor 123 included in the circuit 120, and the transistor133 included in the circuit 130 is turned on. The other of drain andsource electrodes of each of these transistors is connected to the GNDelectrode, so that the potential of one of source and drain electrodesof each of these transistors goes into the L level. Therefore, the gateelectrodes of the transistor 111, the transistor 121, and the transistor131 connected to the one of the source and drain electrodes of thetransistors 111, 121, and 131 go into the L level, whereby thesetransistors are turned off. Thus, in the period T4, the signal OUT1 canbe kept in the L level event when the signal CK1 goes into the H level.Further, as in the period T1, since the signal CK1 is in the H level,the transistor 126 included in the circuit 120 is ON, the signal OUT2 isin the L level.

In the period T5 and the period T6, as in the period T4, in the casewhere the signal IN2 is in the H level, since the transistor 111, thetransistor 121, and the transistor 131 are off, the signal OUT2 and thesignal OUT3 can be kept in the L level even when the signal CK2 and thesignal CK3 are in the H level. In addition, at that time, the signalOUT4, the signal OUT5, and the signal OUT6 outputted from the circuit100 in the second stage sequentially go into the H level as in the casewhere the signal IN1 are inputted to the circuit 100 in the first stage.Further, the transistor 136 included in the circuit 130 is ON in theperiod T5 and the transistor 116 included in the circuit 110 is ON inthe period T6, so that the signal OUT3 and the signal OUT1 go into the Llevel.

As thus described, the signal OUT1, the signal OUT2, and the signal OUT3go into the L level by turning on the transistor 116 included in thecircuit 110, the transistor 126 included in the circuit 120, and thetransistor 136 included in the circuit 130; so that a defect in scanline selection can be suppressed.

Embodiment 6

This embodiment describes another example of a semiconductor device inwhich a plurality of signals can be obtained from one signal. In thisembodiment, a connection relation between a transistor and a signal INand a connection relation between the transistor and a signal CK inEmbodiment 3 are switched.

First, a structure of a semiconductor device of this embodiment will bedescribed with reference to FIG. 9.

The circuit 100 includes the circuit 110, the circuit 120, and thecircuit 130. The circuit 110 includes the transistor 111, the circuit112, and the capacitor 114. The circuit 120 includes the transistor 121,the circuit 122, and the capacitor 124. The circuit 130 includes thetransistor 131, the circuit 132, and the capacitor 134. A signal IN, asignal CK1, a signal CK2, a signal CK3, a signal OUT1, a signal OUT2,and signal OUT3 are transmitted through the wiring 140, the wiring 141,the wiring 142, the wiring 143, the wiring 151, the wiring 152, and thewiring 153, respectively.

Next, a connection relation will be described.

The circuit 100 is connected to the wiring 140, the wiring 141, thewiring 142, and the wiring 143. The circuit 110 is connected to thewiring 140, the wiring 141, and the wiring 151. The circuit 120 isconnected to the wiring 140, the wiring 142, and the wiring 152. Thecircuit 130 is connected to the wiring 140, the wiring 143, and thewiring 153. That is, the wiring 140 is connected to each of the circuit110, the circuit 120, and the circuit 130.

Next, operation of a semiconductor device in FIG. 9 is described withreference to the timing chart in FIG. 6B.

The timing chart in FIG. 6B has the period T1, the period T2, and theperiod T3. The signal IN is an input signal of the circuit 100. Thesignal CK1, the signal CK2, and the signal CK3 are input signals to thecircuit 110, the circuit 120, and the circuit 130, respectively. Thesignal OUT1, the signal OUT2, and the signal OUT3 are output signalsfrom the circuit 110, the circuit 120, and the circuit 130,respectively.

First, operation of the semiconductor device in FIG. 9 in the period T1is described. In the period T1, the signal IN is in the H level, thesignal CK1 is in the H level, the signal CK2 is in the L level, and thesignal CK3 is in the L level. Then, the transistor UI included in thecircuit 110 is turned on; therefore, the signal OUT1 is in the H level,the signal OUT2 is in the L level, and the signal OUT3 is in the Llevel.

Next, in the period T2, the signal IN is in the H level, the signal CK1is in the L level, the signal CK2 is in the H level, and the signal CK3is in the L level. Then, the transistor 121 included in the circuit 120is turned on; therefore, the signal OUT1 is in the L level, the signalOUT2 is in the H level, and the signal OUT3 is in the L level.

Then, in the period T3, the signal IN is in the H level, the signal CK1is in the L level, the signal CK2 is in the L level, and the signal CK3is in the H level. Then, the transistor 131 included in the circuit 130is turned on; therefore, the signal OUT1 is in the L level, the signalOUT2 is in the L level, and the signal OUT3 is in the H level.

Then, the signal OUT1, the signal OUT2, and the signal OUT3 outputtedfrom the circuit 110, the circuit 120, and the circuit 130,respectively, are inputted as scan line selection signals from a scanline driver circuit to corresponding scan lines.

In this manner, a plurality of signals can be obtained from the signalIN. At this time, capacitive coupling of the capacitor 114, thecapacitor 124, and the capacitor 134 included in the circuit 110, thecircuit 120, and the circuit 130, respectively, makes the amplitude ofthe signal OUT1, the signal OUT2, and the signal OUT3 the same as thatof the signal CK1, the signal CK2, and the signal CK3, respectively.

Further, each of potentials of respective gate electrodes of thetransistor 111, the transistor 121, and the transistor 131 is increasedby bootstrap operation in accordance with the respective signals OUT1,OUT2, and OUT3. That is, Vgs of a transistor can be increased or keptlarge, so that distortion of the signals OUT1 to OUT3 is decreased.Alternatively, rising times or falling times of the signals OUT1 to OUT3can be shortened.

In addition, since a signal which has larger amplitude than the signalIN or a power supply voltage is not additionally needed, powerconsumption can be reduced.

Embodiment 7

This embodiment describes a specific example of Embodiment 4.

First, a structure of a semiconductor device of this embodiment will bedescribed with reference to FIG. 10.

The circuit 100 includes the circuit 110, the circuit 120, and thecircuit 130. The circuit 110 includes the transistor 111, the transistor113, the transistor 115, and the capacitor 114. The circuit 120 includesthe transistor 121, the transistor 123, the transistor 125, and thecapacitor 124. The circuit 130 includes the transistor 131, thetransistor 133, the transistor 135, and the capacitor 134. A signal IN,a signal CK1, a signal CK2, a signal CK3, a signal OUT1, a signal OUT2,and signal OUT3 are transmitted through the wiring 140, the wiring 141,the wiring 142, the wiring 143, the wiring 151, the wiring 152, and thewiring 153, respectively.

Next, a connection relation will be described.

The circuit 100 is connected to the wiring 140, the wiring 141, thewiring 142, and the wiring 143. The circuit 110 is connected to thewiring 140, the wiring 141, and the wiring 151. The circuit 120 isconnected to the wiring 140, the wiring 142, and the wiring 152. Thecircuit 130 is connected to the wiring 140, the wiring 143, and thewiring 153. That is, the wiring 140 is connected to each of the circuit110, the circuit 120, and the circuit 130.

The gate electrode of the transistor 111 included in the circuit 110 isconnected to one of the source and drain electrodes of the transistor115 and one of electrodes of the capacitor 114. One of the source anddrain electrodes of the transistor 111 is connected to the wiring 140.The other of the source and drain electrodes of the transistor 111 isconnected to the wiring 151, and the other of the electrodes of thecapacitor 114. The gate electrode of the transistor 115 is connected tothe wiring 141 and the other of the source and drain electrodes of thetransistor 115. The one of the source and drain electrodes of thetransistor 115 is connected to the gate electrode of the transistor 111and the one of the electrodes of the capacitor 114. The gate electrodeof the transistor 113 is connected to the wiring 142. One of the sourceand drain electrodes of the transistor 113 is connected to the gateelectrode of the transistor 111, the one of the source and drainelectrodes of the transistor 115, and the one of the electrodes of thecapacitor 114. The other of the source and drain electrodes of thetransistor 113 is connected to the GND (ground) electrode.

The gate electrode of the transistor 121 included in the circuit 120 isconnected to one of the source and drain electrodes of the transistor125 and one of electrodes of the capacitor 124. One of the source anddrain electrodes of the transistor 121 is connected to the wiring 140.The other of the source and drain electrodes of the transistor 121 isconnected to the wiring 152, and the other of the electrodes of thecapacitor 124. The gate electrode of the transistor 125 is connected tothe wiring 142 and the other of the source and drain electrodes of thetransistor 125. The one of the source and drain electrodes of thetransistor 125 is connected to the gate electrode of the transistor 121and the one of the electrodes of the capacitor 124. The gate electrodeof the transistor 123 is connected to the wiring 143. One of the sourceand drain electrodes of the transistor 123 is connected to the gateelectrode of the transistor 121, the one of the source and drainelectrodes of the transistor 125, and the one of the electrodes of thecapacitor 124. The other of the source and drain electrodes of thetransistor 123 is connected to the GND (ground) electrode.

The gate electrode of the transistor 131 included in the circuit 130 isconnected to one of the source and drain electrodes of the transistor135 and one of electrodes of the capacitor 134. One of the source anddrain electrodes of the transistor 131 is connected to the wiring 140.The other of the source and drain electrodes of the transistor 131 isconnected to the wiring 153, and the other of the electrodes of thecapacitor 134. The gate electrode of the transistor 135 is connected tothe wiring 143 and the other of the source and drain electrodes of thetransistor 135. The one of the source and drain electrodes of thetransistor 135 is connected to the gate electrode of the transistor 131and the one of electrodes of the capacitor 134. The gate electrode ofthe transistor 133 is connected to the wiring 141. One of the source anddrain electrodes of the transistor 133 is connected to the gateelectrode of the transistor 131, the one of the source and drainelectrodes of the transistor 135, and the one of the electrodes of thecapacitor 134. The other of the source and drain electrodes of thetransistor 133 is connected to the GND (ground) electrode.

Next, operation of the semiconductor device in FIG. 10 is described withreference to the timing chart in FIG. 6B.

The timing chart in FIG. 6B has the period T1, the period T2, and theperiod T3. The signal IN is an input signal of the circuit 100. Thesignal CK1, the signal CK2, and the signal CK3 are input signals to thecircuit 110, the circuit 120, and the circuit 130, respectively. Thesignal OUT1, the signal OUT2, and the signal OUT3 are output signalsfrom the circuit 110, the circuit 120, and the circuit 130,respectively. The signal OUT1, the signal OUT2, and the signal OUT3 areinputted as scan line selection signals from a scan line driver circuitto corresponding scan lines.

First, operation of the semiconductor device in FIG. 10 in the period T1is described. In the period T1, the signal IN is in the H level, thesignal CK1 is in the H level, the signal CK2 is in the L level, and thesignal CK3 is in the L level. Then, the transistor 111 included in thecircuit 110 is turned on; therefore, the signal OUT1 is in the H level,the signal OUT2 is in the L level, and the signal OUT3 is in the Llevel. At that time, the transistor 111 included in the circuit 110 iskept on.

Next, in the period T2, the signal IN1 is in the H level, the signal CK1is in the L level, the signal CK2 is in the H level, and the signal CK3is in the L level. Then, the transistor 121 included in the circuit 120is turned on; therefore, the signal OUT1 is in the L level, the signalOUT2 is in the H level, and the signal OUT3 is in the L level. At thattime, the transistor 113 included in the circuit 110 is turned on. Sincethe other of the source and drain electrodes of the transistor 113 isconnected to the GND electrode, the potential of one of the source anddrain electrodes of the transistor 113 goes into the L level. Therefore,the gate electrode of the transistor 111 connected to one of the sourceand drain electrodes of the transistor 113 goes into the L level,whereby the transistor 111 is turned off. Thus, the signal OUT1 can bekept in the L level even when the signal IN is in the H level in theperiod T2. Further, the transistor 121 included in the circuit 120 iskept on.

Then, in the period T3, the signal IN is in the H level, the signal CK1is in the L level, the signal CK2 is in the L level, and the signal CK3is in the H level. Then, the transistor 131 included in the circuit 130is turned on; therefore, the signal OUT1 is in the L level, the signalOUT2 is in the L level, and the signal OUT3 is in the H level. At thattime, the transistor 123 included in the circuit 120 is turned on. Sincethe other of the source and drain electrodes of the transistor 123 isconnected to the GND electrode, the potential of one of the source anddrain electrodes of the transistor 123 goes into the L level. Therefore,the gate electrode of the transistor 121 connected to one of the sourceand drain electrodes of the transistor 123 goes into the L level,whereby the transistor 121 is turned off. Thus, the signal OUT2 can bekept in the L level even when the signal IN is in the H level in theperiod T3. Further, the transistor 131 included in the circuit 130 iskept on.

Similarly, when the period proceeds to the next period, the transistor131 is turned off with use of the signal CK1, so that the signal OUT3can be kept in the L level.

In the case where a structure where the transistor 111, the transistor121, and the transistor 131 are not turned off is used, the signal OUT1,the signal OUT2, and the signal OUT3 are in the H level during thesignal IN is in the H level, which sometimes cause a defect in scan lineselection.

Embodiment 8

This embodiment describes another specific example of Embodiment 4.

First, a structure of a semiconductor device of this embodiment will bedescribed with reference to FIG. 11.

The circuit 100 includes the circuit 110, the circuit 120, and thecircuit 130. The circuit 110 includes the transistor 111, the transistor113, the transistor 115, the transistor 116, and the capacitor 114. Thecircuit 120 includes the transistor 121, the transistor 123, thetransistor 125, the transistor 126, and the capacitor 124. The circuit130 includes the transistor 131, the transistor 133, the transistor 135,the transistor 136, and the capacitor 134. A signal IN, a signal CK1, asignal CK2, a signal CK3, a signal OUT1, a signal OUT2, and signal OUT3are transmitted through the wiring 140, the wiring 141, the wiring 142,the wiring 143, the wiring 151, the wiring 152, and the wiring 153,respectively.

Next, connection relation is described.

The circuit 100 is connected to the wiring 140, the wiring 141, thewiring 142, and the wiring 143. The circuit 110 is connected to thewiring 140, the wiring 141, and the wiring 151. The circuit 120 isconnected to the wiring 140, the wiring 142, and the wiring 152. Thecircuit 130 is connected to the wiring 140, the wiring 143, and thewiring 153. That is, the wiring 140 is connected to each of the circuit110, the circuit 120, and the circuit 130.

The gate electrode of the transistor 111 included in the circuit 110 isconnected to one of the source and drain electrodes of the transistor115 and one of the electrodes of the capacitor 114. One of the sourceand drain electrodes of the transistor 111 is connected to the wiring140. The other of the source and drain electrodes of the transistor 111is connected to the wiring 151, one of the source and drain electrodesof the transistor 116, and the other of the electrodes of the capacitor114. The gate electrode of the transistor 115 is connected to the wiring141 and the other of the source and drain electrodes of the transistor115. The one of the source and drain electrodes of the transistor 115 isconnected to the gate electrode of the transistor 111 and the one of theelectrodes of the capacitor 114. The gate electrode of the transistor113 is connected to the wiring 142. One of the source and drainelectrodes of the transistor 113 is connected to the gate electrode ofthe transistor 111, the one of the source and drain electrodes of thetransistor 115, and the one of the electrodes of the capacitor 114. Theother of the source and drain electrodes of the transistor 113 isconnected to the GND (ground) electrode. The gate electrode of thetransistor 116 is connected to the wiring 143. The one of the source anddrain electrodes of the transistor 116 is connected to the wiring 151,the other of the source and drain electrodes of the transistor 111, andthe other of the electrodes of the capacitor 114. The other of thesource and drain electrodes of the transistor 116 is connected to theGND (ground) electrode.

The gate electrode of the transistor 121 included in the circuit 120 isconnected to one of the source and drain electrodes of the transistor125 and one of electrodes of the capacitor 124. One of the source anddrain electrodes of the transistor 121 is connected to the wiring 140.The other of the source and drain electrodes of the transistor 121 isconnected to the wiring 152, one of the source and drain electrodes ofthe transistor 126, and the other of the electrodes of the capacitor124. The gate electrode of the transistor 125 is connected to the wiring142 and the other of the source and drain electrodes of the transistor125. The one of the source and drain electrodes of the transistor 125 isconnected to the gate electrode of the transistor 121 and the one of theelectrodes of the capacitor 124. The gate electrode of the transistor123 is connected to the wiring 143. One of the source and drainelectrodes of the transistor 123 is connected to the gate electrode ofthe transistor 121, the one of the source and drain electrodes of thetransistor 125, and the one of the electrodes of the capacitor 124. Theother of the source and drain electrodes of the transistor 123 isconnected to the GND (ground) electrode. The gate electrode of thetransistor 126 is connected to the wiring 141. The one of the source anddrain electrodes of the transistor 126 is connected to the wiring 152,the other of the source and drain electrodes of the transistor 121, andthe other of the electrodes of the capacitor 124. The other of thesource and drain electrodes of the transistor 126 is connected to theGND (ground) electrode.

The gate electrode of the transistor 131 included in the circuit 130 isconnected to one of the source and drain electrodes of the transistor135 and one of electrodes of the capacitor 134. One of the source anddrain electrodes of the transistor 131 is connected to the wiring 140.The other of the source and drain electrodes of the transistor 131 isconnected to the wiring 153, one of the source and drain electrodes ofthe transistor 136, and the other of the electrodes of the capacitor134. The gate electrode of the transistor 135 is connected to the wiring143 and the other of the source and drain electrodes of the transistor135. The one of the source and drain electrodes of the transistor 135 isconnected to the gate electrode of the transistor 131 and the one of theelectrodes of the capacitor 134. The gate electrode of the transistor133 is connected to the wiring 141. One of the source and drainelectrodes of the transistor 133 is connected to the gate electrode ofthe transistor 131, the one of the source and drain electrodes of thetransistor 135, and the one of the electrodes of the capacitor 134. Theother of the source and drain electrodes of the transistor 133 isconnected to the GND (ground) electrode. The gate electrode of thetransistor 136 is connected to the wiring 142. The one of the source anddrain electrodes of the transistor 136 is connected to the wiring 153,the other of the source and drain electrodes of the transistor 131, andthe other of the electrodes of the capacitor 134. The other of thesource and drain electrodes of the transistor 136 is connected to theGND (ground) electrode.

Next, operation of the semiconductor device in FIG. 11 is described withreference to the timing chart in FIG. 6B.

The timing chart in FIG. 6B has the period T1, the period T2, and theperiod T3. The signal IN is an input signal of the circuit 100. Thesignal CK1, the signal CK2, and the signal CK3 are input signals to thecircuit 110, the circuit 120, and the circuit 130, respectively. Thesignal OUT1, the signal OUT2, and the signal OUT3 are output signalsfrom the circuit 110, the circuit 120, and the circuit 130,respectively.

First, operation of the semiconductor device in FIG. 11 in the period T1is described. In the period T1, the signal IN is in the H level, thesignal CK1 is in the H level, the signal CK2 is in the L level, and thesignal CK3 is in the L level. Then, the transistor 111 included in thecircuit 110 is turned on and the signal OUT1 goes into the H level, thesignal OUT2 goes into the L level, and the signal OUT3 goes into the Llevel. At that time, the transistor 126 included in the circuit 120 isturned on and the signal OUT2 goes into the L level.

Next, in the period T2, the signal IN1 is in the H level, the signal CK1is in the L level, the signal CK2 is in the H level, and the signal CK3is in the L level. Then, the transistor 121 included in the circuit 120is turned on and the signal OUT1 is in the L level, the signal OUT2 isin the H level, and the signal OUT3 is in the L level. At that time, thetransistor 113 included in the circuit 110 is turned on. Since the otherof the source and drain electrodes of the transistor 113 is connected tothe GND electrode, the potential of one of the source and drainelectrodes of the transistor 113 goes into the L level. Therefore, thegate electrode of the transistor 111 connected to one of the source anddrain electrodes of the transistor 113 goes into the L level, wherebythe transistor 111 is turned off. Thus, the signal OUT1 can be kept inthe L level even when the signal IN is in the H level in the period T2.Further, the transistor 136 included in the circuit 130 is turned on, sothat the signal OUT3 goes into L level.

Then, in the period T3, the signal IN is in the H level, the signal CK1is in the L level, the signal CK2 is in the L level, and the signal CK3is in the H level. Then, the transistor 131 included in the circuit 130is turned on and the signal OUT1 is in the L level, the signal OUT2 isin the L level, and the signal OUT3 is in the H level. At that time, thetransistor 123 included in the circuit 120 is turned on. Since the otherof the source and drain electrodes of the transistor 123 is connected tothe GND electrode, the potential of one of the source and drainelectrodes of the transistor 123 goes into the L level. Therefore, thegate electrode of the transistor 121 connected to one of the source anddrain electrodes of the transistor 123 goes into the L level, wherebythe transistor 121 is turned off. Thus, the signal OUT2 can be kept inthe L level even when the signal IN is in the H level in the period T3.Further, the transistor 116 included in the circuit 110 is turned on, sothat the signal OUT1 goes into the L level.

Similarly, when the period proceeds to the next period, the transistor131 is turned off with use of the signal CK1, so that the signal OUT3can be kept in the L level.

As thus described, the signal OUT1, the signal OUT2, and the signal OUT3go into the L level by turning on the transistor 116 included in thecircuit 110, the transistor 126 included in the circuit 120, and thetransistor 136 included in the circuit 130; so that a defect in scanline selection can be suppressed.

Embodiment 9

This embodiment describes a driver circuit to which a structure relatedto an embodiment of this invention is adopted.

First, a structure of a semiconductor device in this embodiment isdescribed with the circuit 100 in FIG. 6A given for example and withreference to FIG. 12.

A shift register 2000 outputs a plurality of signals sequentially. Thecircuits 100 in a first stage to an n-th stage is a circuit related toan embodiment of this invention and, here, each output three signalswhich can be obtained from one signal here. Further, output signals fromthe shift register 2000 is transmitted through the wirings 140 to thecircuits 100 in the first stage to the n-th stage. Signals OUT1 to OUT3are transmitted through n groups of the respective wirings 151 to 153.

Next, a connection relation will be described.

The shift register 2000 is connected to the wiring 140. The circuit 100is connected to the wiring 140, the wiring 151, the wiring 152, and thewiring 153.

Next, operation of the semiconductor device in FIG. 12 is described withreference to a timing chart in FIG. 13.

The timing chart in FIG. 13 shows one frame period of the drivercircuit. A signal SRout1 is an input signal of the circuit 100 in thefirst stage. A signal SRout2 is an input signal of the circuit 100 inthe second stage. A signal SRoutN is an input signal of the circuit 100in the n-th stage. A period with a pulse of these signals SRout1 toSRoutN represents a sub-frame period. A signal CK1, a signal CK2, and asignal CK3 are input signals of each of the circuits 100 in the firststage to the n-th stage. A period with pulses of these signals CK1 toCK3 represents a scan line selection period. Signals OUT1 to OUT3 n areoutput signals of each of the circuit 100 in the first stage to the n-thstage.

A signal OUT goes into the H level only when the signal SRout and thesignal CK which are sequentially inputted both go into the H level. Thatis, when the signals SRout1 to SRoutN are inputted in one frame period,the signals OUT1 to OUT3 n are outputted. Thus, scan lines 1 to 3 n canbe controlled in one frame period. Further, the signal SRout and thesignal CK are inputted and the signal OUT is outputted also in a secondframe and its subsequent frames. In general, moving images are displayedby being processed in 60 frames per second.

Note that in this embodiment, a structure of a circuit which is providedon the output side of the shift register in a scan line driver circuitis described by giving the circuit 100 in FIG. 6A for example. However,the circuit 100 illustrated in FIG. 7A, FIG. 8, FIG. 9, FIG. 10 and FIG.11 can be used for the semiconductor device related to an embodiment ofthis invention.

Embodiment 10

This embodiment describes an example of a cross-sectional structure of adisplay device.

FIG. 14A illustrates an example of a top view of the display device. Adriver circuit portion 5392 and a pixel portion 5393 are formed over asubstrate 5391. An example of the driver circuit portion 5392 is a scanline driver circuit, a signal line driver circuit, or the like.

FIG. 14B illustrates an example of a cross-sectional view of the drivercircuit portion 5392 (a cross section taken along line A-B in FIG. 14A).For example, FIG. 14B illustrates a substrate 5401, a conductive layer5402 a, a conductive layer 54026, an insulating layer 5403, a conductivelayer 5404 a, a conductive layer 5404 b, a semiconductor layer 5405, aninsulating layer 5406, a conductive layer 5407, a liquid crystal layer5408, an insulating layer 5409, a conduction layer 5410, and a substrate5411. For example, the conductive layer 5402 a is formed over thesubstrate 5401. For example, the conductive layer 5402 b is formed overthe conductive layer 5402 a. For example, the insulating layer 5403 isformed over the substrate 5401, the conductive layer 5402 a, and theconductive layer 5402 b. For example, the conductive layer 5404 a isformed over the insulating layer 5403. For example, the conductive layer5404 b is formed over the conductive layer 5404 a. For example, thesemiconductor layer 5405 is formed over the insulating layer 54013. Forexample, the insulating layer 5406 is formed over the insulating layer5403, the conductive layer 5404 a, the conductive layer 5404 b, and thesemiconductor layer 5405. For example, the conductive layer 5407 isformed in an opening portion of the insulating layer 5406 and over theinsulating layer 5406. For example, the liquid crystal layer 5408 isformed over the insulating layer 5406. For example, the insulating layer5409 is formed over the insulating layer 5406 and the conductive layer5407. For example, the conductive layer 5410 is formed over the liquidcrystal layer 5408 and the insulating layer 5409. For example, thesubstrate 5411 is formed over the insulating layer 5410.

FIG. 14C illustrates an example of a cross-sectional view of the pixelportion 5393 (a cross section taken along line C-D in FIG. 14A). Forexample, FIG. 14C illustrates the substrate 5401, the conductive layer5402 a, the insulating layer 5403, the conductive layer 5404 a, thesemiconductor layer 5405, the insulating layer 5406, the conductivelayer 5407, the liquid crystal layer 5408, the conduction layer 5410,and the substrate 5411. For example, the conductive layer 5402 a isformed over the substrate 5401. For example, the insulating layer 5403is formed over the substrate 5401 and the conductive layer 5402 a. Forexample, the conductive layer 5404 a is formed over the insulating layer5403. For example, the semiconductor layer 5405 is formed over theinsulating layer 5403. For example, the insulating layer 5406 is formedover the insulating layer 5403, the conductive layer 5404 a, and thesemiconductor layer 5405. For example, the conductive layer 5407 isformed in an opening portion of the insulating layer 5406 and over theinsulating layer 5406. For example, the liquid crystal layer 5408 isformed over the insulating layer 5406 and the conduction layer 5407. Forexample, the conductive layer 5410 is formed over the liquid crystallayer 5408. For example, the substrate 5411 is formed over theinsulating layer 5410.

For example, the conductive layer 5402 a and the conductive layer 5402 bcan function as gate electrodes or gate wirings. For example, theinsulating layer 5403 can function as a gate insulating layer. Forexample, the conductive layer 5404 a and the conductive layer 5404 b canfunction as wirings, electrodes of a transistor, electrodes of acapacitor, or the like. For example, the insulating layer 5406 canfunction as an interlayer film or a planarizing film. For example, theconductive layer 5407 can function as a wiring, a pixel electrode, alight-transmitting electrode, or a reflective electrode. For example,the insulating layer 5409 can function as a sealing material. Forexample, the conductive layer 5410 can function as a counter electrode,a common electrode, or a reflective electrode.

Here, for example, the conductive layer 5402 a and the conductive layer5404 a can be formed using a light-transmitting material. For example,the conductive layer 5402 b and the conductive layer 5404 b can beformed using a material having higher conductivity material than amaterial used for the conductive layer 5402 a and the conductive layer5404 a. For example, the conductive layer 5402 b and the conductivelayer 5404 b can be formed using a light-blocking material. In thismanner, the resistance of wirings can be reduced in the driver circuitportion 5392. Therefore, power consumption of the driver circuit can bereduced, driving frequency can be high, or a driving voltage can be low.Meanwhile, wirings, electrodes of a transistor, electrodes of a storagecapacitor, and/or the like in the driver circuit portion 5392 cantransmit light. That is, a light-transmitting region (an opening portionof a pixel) can be larger. Therefore, power consumption can be reducedor resolution of the pixel portion can be high. However, an example ofthis embodiment is not limited to this. For example, the conductivelayer 5402 a and the conductive layer 5404 a can be formed using alight-blocking material. Alternatively, in the pixel portion 5393, agate wiring can have a layered structure of the conductive layer 5402 aand the conductive layer 5402 b like a wiring of the driver circuitportion 5392. Moreover, for example, a source wiring can have a layeredstructure of the conductive layer 5404 a and the conductive layer 5404b. In this manner, delay or distortion of a signal (e.g., a video signalor a scan line selection signal) inputted to a pixel can be small. Inanother example, one or both of the conductive layer 5402 a or 5402 band the conductive layer 5404 a or 5404 b can be omitted. In anotherexample, in one or both of a transistor portion of the driver circuitportion 5392 and a transistor portion of the pixel portion 5393, a gateelectrode can have a layered structure of the conductive layer 5402 aand the conductive layer 5402 b. In another example, the conductivelayer 5402 b can be formed under the conductive layer 5402 a. In anotherexample, the conductive layer 5404 b can be formed under the conductivelayer 5404 a. In another example, the semiconductor layer 5405 can beformed over the insulating layer 5403 and the conductive layer 5402 acan be formed over the insulating layer 5403 and the semiconductor layer5405.

Note that, for example, an oxide semiconductor can be used for asemiconductor layer. For example, an oxide semiconductor often has alight-transmitting property. When an oxide semiconductor is combinedwith a display device in this embodiment, the aperture ratio of a pixelcan be improved. However, an example of this embodiment is not limitedto this. For example, for the semiconductor layer, a single crystalsemiconductor, a polycrystalline semiconductor, a microcrystalline(microcrystal or nanocrystal) semiconductor, an amorphous semiconductor,various non-single-crystal semiconductors, or the like can be used.

Note that, for example, a light-emitting element (e.g., an EL element)can be used as a display element. FIG. 15A illustrates an example of across-sectional view of the driver circuit portion 5392 in a displaydevice in which a light-emitting element is used as a display elementfor example. FIG. 15A is different from FIG. 14B in that an insulatinglayer 5412 is formed over the insulating layer 5406 and the conductionlayer 5407, the insulating layer 5409 and a filler 5414 are formed overthe insulating layer 5412, and the like. FIG. 15B illustrates an exampleof a cross-sectional view of the pixel portion 5393 in a display devicein which a light-emitting element is used as a display element. FIG. 15Bis different from FIG. 14C in that the insulating layer 5412 is formedover the insulating layer 5406 and the conduction layer 5407, alight-emitting layer 5413 is formed over an opening portion of theinsulating layer 5412, the conductive layer 5410 is formed over theinsulating layer 5412 and the light-emitting layer 5413, the filler 5414is formed over the conductive layer 5410, and the like. For example, theinsulating layer 5412 can function as a partition wall. However, anexample of this embodiment is not limited to this.

Note that, for example, an element (e.g., an electrophoresis element, aparticle movement element, and electronic liquid powder) in whichparticle moves to perform display can be used as a display element. Insuch a manner, an electronic paper can be manufactured. FIG. 16Aillustrates an example of a cross-sectional view of the driver circuitportion 5392 in a display device in which an electrophoresis element isused as a display element. In a part of the driver circuit portion 5392,an electrophoresis element is provided between the insulating layer 5406and the conductive layer 5410. In addition, the insulating layer 5409 isformed so as to cover the electrophoresis element. FIG. 16B illustratesan example of a cross-sectional view of the pixel portion 5393 in adisplay device in which an electrophoresis element is used as a displayelement. The electrophoresis element is provided between the conductivelayer 5407 and the conductive layer 5410. Note that, for example, theelectrophoresis element includes a capsule 5415, liquid 5416, particles5417, and particles 5418. The liquid 5416, the particles 5417, and theparticles 5418 are in the capsule 5415 for example. For example, theliquid 5416 often has an insulating property and a light-transmittingproperty. One of the particles 5417 and the particles 5418 is positivelycharged and the other thereof is negatively charged in many cases. Oneof the particles 5417 and the particles 5418 is white and the otherthereof is black in many cases. However, an example of this embodimentis not limited to this. For example, colors of the particles 5417 andthe particles 5418 are not limited to white or black, and differentcolors can be used (e.g., red, green, blue, magenta, yellow, and cyan).

In the display device in this embodiment, an aperture rate of the pixelcan be improved while performance of the driver circuit is improved.Further, when the structures described in Embodiments 3 to 9 are usedfor the driver circuit, power consumption can be reduced, drivingfrequency is improved, and resolution of the pixel portion can be high.

Embodiment 11

This embodiment describes examples of electronic devices.

FIGS. 17A to 17H and FIGS. 18A to 18D illustrate electronic devices.These electronic devices can include a housing 5000, a display portion5001, a speaker 5003, an LED lamp 5004, operation keys 5005 (including apower switch or operation switch), a connection terminal 5006, a sensor5007 (a sensor having a function of measuring force, displacement,position, speed, acceleration, angular velocity, rotational frequency,distance, light, liquid, magnetism, temperature, chemical substance,sound, time, hardness, electric field, current, voltage, electric power,radiation, flow rate, humidity, gradient, oscillation, odor, or infraredray), a microphone 5008, and the like.

FIG. 17A illustrates a mobile computer which can include a switch 5009,an infrared port 5010, and the like in addition to the above objects.FIG. 17B illustrates a portable image reproducing device (e.g., a DVDreproducing device) provided with a memory medium, which can include asecond display portion 5002, a memory medium reading portion 5011, andthe like in addition to the above objects. FIG. 17C illustrates agoggle-type display which can include the second display portion 5002, asupporting portion 5012, an earphone 5013, and the like in addition tothe above objects. FIG. 17D illustrates a portable game machine whichcan include the memory medium reading portion 5011 and the like inaddition to the above objects. FIG. 17E illustrates a projector whichcan include a light source 5033, a projection lens 5034, and the like inaddition to the above objects. FIG. 17F illustrates a portable gamemachine which can include the second display portion 5002, the memorymedium reading portion 5011, and the like in addition to the aboveobjects. FIG. 17G illustrates a television receiver which can include atuner, an image processing portion, and the like in addition to theabove objects. FIG. 17H illustrates a portable television receiver whichcan include a charger 5017 which can transmit and receive signals andthe like in addition to the above objects. FIG. 18A illustrates adisplay which can include a supporting board 5018 and the like inaddition to the above objects. FIG. 18B illustrates a camera which caninclude an external connecting port 5019, a shutter button 5015, animage receiver portion 5016, and the like in addition to the aboveobjects. FIG. 18C illustrates a computer which can include a pointingdevice 5020, the external connecting port 5019, a reader/writer 5021,and the like in addition to the above objects. FIG. 18D illustrates amobile phone which may include an antenna 5014, a tuner of 1 seg(one-segment partial reception service for mobile phones and mobileterminals), and the like in addition to the above objects.

The electronic devices shown in FIGS. 17A to 17H and FIGS. 18A to 18Dcan have a variety of functions. For example, a function of displaying avariety of information (a still image, a moving image, a text image, andthe like) on a display portion, a touch panel function, a function ofdisplaying a calendar, date, time, and the like, a function forcontrolling a process with a variety of software (programs), a wirelesscommunication function, a function of being connected to a variety ofcomputer networks with a wireless communication function, a function oftransmitting and receiving a variety of data with a wirelesscommunication function, a function of reading program or data stored ina memory medium and displaying the program or data on a display portion,and the like can be given. Further, the electronic device including aplurality of display portions can have a function of displaying imageinformation mainly on one display portion while displaying textinformation on another display portion, a function of displaying athree-dimensional image by displaying images where parallax isconsidered on a plurality of display portions, or the like. Furthermore,the electronic device including an image receiver portion can have afunction of shooting a still image, a function of shooting a movingimage, a function of automatically or manually correcting a shot image,a function of storing a shot image in a memory medium (an externalmemory medium or a memory medium incorporated in the camera), a functionof displaying a shot image on the display portion, or the like. Notethat functions which the electronic devices can include illustrated inFIGS. 17A to 17H and FIGS. 18A to 18D are not limited thereto, and theelectronic devices can have a variety of functions.

The electronic devices described in this embodiment each include thedisplay portion for displaying some sort of information. By acombination of the electronic device in this embodiment and thesemiconductor device, shift register, or display device in Embodiments 1to 4, improvement in reliability, improvement in yield, reduction incost, increase in the size of the display portion, increase in thedefinition of the display portion, or the like can be achieved.

Next, applications of a semiconductor device will be described.

FIG. 18E illustrates an example in which a semiconductor device isprovided so as to be integrated with a building. FIG. 18E illustrates ahousing 5022, a display portion 5023, a remote controller device 5024which is operation portion, a speaker 5025, and the like. Thesemiconductor device is incorporated in the building as a wall-hangingtype, so that the semiconductor device can be provided without requiringa wide space.

FIG. 18F illustrates another example in which a semiconductor device isprovided so as to be integrated within a building. The display panel5026 is integrated with a prefabricated bath 5027, so that a person whotakes a bath can watch the display panel 5026.

Note that although this embodiment gives the wall and the prefabricatedbath as examples of the building, this embodiment is not limited to themand the semiconductor device can be provided in a variety of buildings.

Next, an example in which the semiconductor device is provided so as tobe integrated with a moving body will be described.

FIG. 18G illustrates an example in which the semiconductor device isprovided in a vehicle. A display panel 5028 is provided in a body 5029of the vehicle and can display information inputted from the operationof the body or the outside of the body on demand. Note that a navigationfunction may be provided.

FIG. 18H illustrates an example in which the semiconductor device isprovided so as to be integrated with a passenger airplane. FIG. 18Hillustrates a usage pattern when a display panel 5031 is provided on aceiling 5030 above a seat in the passenger airplane. The display panel5031 is integrated with the ceiling 5030 through a hinge portion 5032,and a passenger can watch the display panel 5031 by extending andcontracting the hinge portion 5032. The display panel 5031 has afunction of displaying information when it is operated by the passenger.

Note that although this embodiment gives the body of the vehicle and thebody of the plane as examples of the moving body, this embodiment is notlimited to these examples. The display device can be provided for avariety of moving bodies such as a two-wheel motor vehicle, a four-wheelvehicle (including a car, bus, and the like), a train (including amonorail, a railway, and the like), and a ship.

This application is based on Japanese Patent Application serial No.2009-172949 filed with Japan Patent Office on Jul. 24, 2009, the entirecontents of which are hereby incorporated by reference.

The invention claimed is:
 1. A semiconductor device comprising: aplurality of pixels, each including a display element and a firsttransistor; and a scan line driver circuit supplying a signal to a scanline, wherein the scan line driver circuit includes a second transistor,a capacitor holding a voltage between a gate of the second transistorand a source of the second transistor, and a third transistorcontrolling an electrical connection between the gate of the secondtransistor and a ground, wherein the source of the second transistor iselectrically connected to the scan line and the capacitor, wherein afirst signal is configured to be input to the gate of the secondtransistor, and a second signal is configured to be input to a gate ofthe third transistor, and wherein, when the semiconductor device isoperated, the first signal and the second signal are never bothsimultaneously provided at a low level.
 2. The semiconductor deviceaccording to claim 1, wherein the first transistor includes an oxidesemiconductor layer.
 3. The semiconductor device according to claim 1,wherein a gate electrode layer of the second transistor, a sourceelectrode layer of the second transistor, and a drain electrode layer ofthe second transistor contain a light-transmitting conductive film, andwherein the second transistor includes an oxide semiconductor layer. 4.The semiconductor device according to claim 1, wherein the semiconductordevice is incorporated in one selected from the group consisting of adisplay device, a computer, an image reproducing device, a goggle-typedisplay, a game machine, a projector, a television receiver, a cameraand a mobile phone.
 5. A semiconductor device comprising: a plurality ofpixels, each including a display element and a first transistor; and ascan line driver circuit supplying a signal to a scan line, wherein apixel electrode layer of the display element, a gate electrode layer ofthe first transistor, a source electrode layer of the first transistor,a drain electrode layer of the first transistor, and the scan linecontain a light-transmitting conductive film, wherein the scan linedriver circuit includes a second transistor, a capacitor holding avoltage between a gate of the second transistor and a source of thesecond transistor, a third transistor controlling an electricalconnection between the gate of the second transistor and a ground,wherein the source of the second transistor is electrically connected tothe scan line and the capacitor, wherein a first signal is configured tobe input to the gate of the second transistor, and a second signal isconfigured to be input to a gate of the third transistor, and wherein,when the semiconductor device is operated, the first signal and thesecond signal are never both simultaneously provided at a low level. 6.The semiconductor device according to claim 5, wherein thelight-transmitting conductive film is formed using a sputtering targetcontaining metal oxide.
 7. The semiconductor device according to claim5, wherein the first transistor includes an oxide semiconductor layer.8. The semiconductor device according to claim 5, wherein a gateelectrode layer of the second transistor, a source electrode layer ofthe second transistor, and a drain electrode layer of the secondtransistor contain the light-transmitting conductive film, and whereinthe second transistor includes an oxide semiconductor layer.
 9. Thesemiconductor device according to claim 5, wherein the semiconductordevice is incorporated in one selected from the group consisting of adisplay device, a computer, an image reproducing device, a goggle-typedisplay, a game machine, a projector, a television receiver, a cameraand a mobile phone.
 10. The semiconductor device according to claim 5,wherein the light-transmitting conductive film includes gallium.